Since there is no complete in-game glossary of Status Effects and Enemy's actions, this is my personal compilation of all the icons I have encountered and what they mean.
To search through the guide, use Ctrl+F.

Disclaimer: I have no data extraction expertise, so these are all screenshots. I do my best to clean up the icons, but apologies for the scuff. Feel free to correct me if you find any incorrect information.

While in battle, press “R” (on keyboard) or “RB” (on Xbox controller) to see the Status Effects of your bullets.
To see the Status Effects of enemies, press “R” or “RB”, select a bullet, then target the enemy as if you are going to attack.

You're probably here because of a few specific icons that are never explained in-game.
Here are the ones I was initially frustrated or confused by.

Used by: Enemies

Effect: Summon a number of enemies based on the number on its right.

Used by: Player and Enemies

Player's Effect: Negates Debuffs being applied. (Stacks) Each stack is consumed to negate one debuff.
Enemy's Effect: Negates Debuffs being applied. (Stacks) Each stack is consumed to negate one debuff.

Note: Does not Cure debuffs that were applied before Antibody.

Used by: Enemies

Effect: Transforms, changing the skills this enemy can use.

Used by: Enemies

Effect: Moves to any empty position.

Used by: Enemies

Effect: Covers most of your hand in ink and prevents you from seeing, hearing, and reading about your bullets. (Stacks) Each stack extends its duration.

Note: A bit of the bottom is still visible, letting you see your bullets' cartridges.

Used by: Enemies

Effect: Increases the Cost of all bullets by the amount indicated in the icon. (Stacks) Each stack extends its duration.

Used by: Enemies

Effect: Reduces the player's Shield based on the number to its right.

Note: Does not reduce Armor.

Used by: Enemies

Effect: The enemy does nothing for one turn.

Used by: Player and Enemies

Player's Effect: Remove Debuffs from target.
Enemy's Effect: Remove Debuffs from target.

Used by: Enemies

Effect: Deal a percentage of damage received back to the attacker. (25% or 30%)

Note 1: Reflect damage is inflicted before your bullet's other effects, such as Shield, Armor, and Heal.
Note 2: Reflect damage is rounded down, to a minimum of 1 damage.
Note 3: Damage exceeding enemy's HP counts towards Reflect damage.
Note 4: There are two versions of Reflect (25% and 30%). You can check by pressing "R" or "RB", select a bullet, then target the enemy as if you are going to attack.

Used by: Player and Enemies

Player's Effect: Removes certain Buffs from the target, such as Shield and Armor, but not Attack Up. (Further testing required to find to full list of dispellable buffs.)
Enemy's Effect: Removes one type of Buff from the player, either Shield and Armor.

Note: You can check which type of Dispel the enemy is casting by pressing "R" or "RB", select a bullet, then target the enemy as if you are going to attack.

Used by: Enemies

Effect: Deals damage to itself and the player according to the number to its right.

Effect: Damages the target based on the amount on its right. Shield is damaged first if the target has Shield, followed by Armor if the target has Armor.

Effect: Gains Shield based on the amount on its right. Shield is damaged first if the player/enemy has shield.

Note: The player's Shield expires after an enemy's turn. The enemy's Shield disappears after its own turn.

Effect: Gains Armor based on the amount on its right. Armor is damaged if the player has armor, blocking Attack.

Note 1: Shield is damaged before Armor.
Note 2: Armor persists even after the enemy's turn.

Effect: Recovers HP based on the amount on its right.

A list of Status Effects and Actions used by both the player and enemies. Sorted according to the order you will (probably) discover them.

Player's Effect: Increases damage by a certain amount.
Enemy's Effect: Increases damage based on the amount on its right. The buff persists even after the enemy's turn but is consumed on its next attack.

Player's Effect: Decreases damage based on the amount next to its right. The debuff disappears after you attack.
Enemy's Effect: Decreases damage by the amount next to the icon. The debuff expires after its next Attack, but persists if it does not Attack.

Player's Effect: Discard a certain number of bullets.
Enemy's Effect: Forces you to discard a random bullet.

Player's Effect: Recovers HP by 3% of maximum HP at the start of the turn. (Stacks) Each stack extends its duration.
Enemy's Effect: Recovers HP by 3% of maximum HP at the start of the turn. (Stacks) Each stack extends its duration.

Note: Recovery value is rounded down.

Player's Effect: Damages HP by 3% of maximum HP at the start of the turn. (Stacks) Each stack extends its duration.
Enemy's Effect: Damages HP by 3% of maximum HP at the start of the turn. (Stacks) Each stack extends its duration.

Note: Damage value is rounded down.

Player's Effect: Ignores Shield and Armor when inflicting damage.
Enemy's Effect: Ignores Shield and Armor when inflicting damage.

Player's Effect: Takes no damage.
Enemy's Effect: Takes no damage.

Note: Might stack. (Requires testing.)

Player's Effect: Negates Debuffs being applied. (Stacks) Each stack is consumed to negate one debuff.
Enemy's Effect: Negates Debuffs being applied. (Stacks) Each stack is consumed to negate one debuff.

Note: Does not Cure debuffs that were applied before Antibody.

Player's Effect: Removes all Debuffs from target.
Enemy's Effect: Removes all Debuffs from target.

Player's Effect: Removes certain Buffs from the target, such as Shield and Armor, but not Attack Up. (Further testing required to find to full list of dispellable buffs.)
Enemy's Effect: Removes one type of Buff from the player, either Shield and Armor.

Note: You can check which type of Dispel the enemy is casting by pressing "R" or "RB", select a bullet, then target the enemy as if you are going to attack.

List of Status Effects and Actions only used by enemies. Sorted according to the order you will (probably) discover them.

Effect: Summon a number of enemies based on the amount on its right.

Effect: Covers most of your hand in ink and prevents you from seeing, hearing, and reading about your bullets. (Stacks) Each stack extends its duration.

Note: A bit of the bottom is still visible, letting you see your bullets' cartridges.

Effect: Transforms, changing the skills this enemy can use.

Effect: Moves to any empty position.

Effect: Increases the Cost of all bullets by the amount indicated in the icon. (Stacks) Each stack extends its duration.

Effect: Reduces the player's Shield based on the number to its right.

Note: Does not reduce Armor.

Effect: The enemy does nothing for one turn.

Effect: Deal a percentage of damage received back to the attacker. (25% or 30%)

Note 1: Reflect damage is inflicted before your bullet's other effects, such as Shield, Armor, and Heal.
Note 2: Reflect damage is rounded down, to a minimum of 1 damage.
Note 3: Damage exceeding enemy's HP counts towards Reflect damage.
Note 4: There are two versions of Reflect (25% and 30%). You can check by pressing "R" or "RB", select a bullet, then target the enemy as if you are going to attack.

Effect: Deals damage to itself and the player according to the number to its right.

List of Status Effects and actions only used by the player. Excluding Buffs unique to just one bullet. Sorted according to the order you will (probably) discover them.

Effect: If you build the Heartsplosion meter to 100, you'll enter a state that halves the Cost of all "Seasons" bullets, and Delays all enemies by 10.
The state will end upon ending your turn, and the Heartsplosion meter will reset to 0.

Effect: Deals 30 damage to a random enemy for every 10 Combo Beam stacks you rack up.
Maximum stack of 999.

Effect: Deals 3 damage to a random enemy at the end of your turn.

Effect: Gain 3 Shield at the end of your turn.

Effect: Recovers 2 HP at the end of your turn.

Effect: Activates the effects of all active drones.

Effect: Lowers bullet Cost.

Effect: When you attack, take damage based on the amount on its right.

Effect: Increases damage dealt based on: the number of bullets used this turn, multiplied by the number in its name (eg. Burst 1, Burst 7).

Effect: Draw a number of bullets from your magazine.

Note: You can only hold a maximum of 7 bullets. Excess Draw will not be used; your hand will remain the same.

Effect: Discard all bullets in your hand, then Draw as many as you discarded.

Note: You can only hold a maximum of 7 bullets. Excess Draw will not be used; your hand will remain the same.

Effect: Increases target's wait time.

Effect: Prevents HP recovery. (Stacks) Each stack extends its duration.

Note: Also prevents Regen.

Effect: Removes the bullet from this battle; it will not be in the graveyard and will not appear until the battle is over.

Effect: The target will do nothing. (Stacks) Each stack extends its duration.

Note: The target will keep trying to do the same actions. The target's wait time will be the same each turn until it is able to act.

Effect: Kills the target when its Countdown stacks reaches 0. (Stacks) Each stack extends its duration, delaying its instant death effect.

Note 1: Stack decreases by 1 at the end of any enemy's turn, even if this enemy has not had its turn yet.
Note 2: Does not work on bosses.

Effect: When the target attacks, 50% chance to hit itself.

Effect: Target cannot gain Shield or Armor. (Stacks) Each stack increases its duration.

List of unique Buffs from specific bullets. Sorted in alphabetical order.

Effect: Deal 3 damage to a random enemy each time you build your Heartsplosion meter.

Effect: Combo Beam +1 at the end of your turn.

Effect: Deal 4 damage to a random enemy each time you discard a bullet.

Effect: Gain 3 Armor each time you gain Combo Beam.

Effect: Gain additional +2 Shield from Shield Drones.

Effect: Attack Drones deals 2 more damage.

Effect: Deal 5 damage to a random enemy each time you gain Armor.

Effect: Recover additional +1 HP from Healing Drones.

Effect: Deal 2 damage to a random enemy each time you recover HP.

Effect: Deal 3 damage to a random enemy each time you gain Shield.

Effect: Combo Beam attacks all enemies.

Effect: Deal 2 damage to all enemies each time you discard a bullet.

Effect: Increases the damage of Poison from 3% to 5% of maximum HP. (Only against enemies.)

Effect: Recover 4 HP at the end of your turn.

Effect: Gain 7 Shield at the end of your turn.

Effect: All bullets have -2 Cost.

Effect: All bullets have +3 Attack.

Effect: All bullets have -1 Cost and +1 Attack. Recover 1 HP and gain 3 Shield at the end of your turn.