Primary Attack
The main attack of your unit. Every unit has one.
The primary attack is not nescessarilly the first attack that a stack does in a battle: if your unit also has an Extra Attack, and that is of higher initiative, that one is done first.
A Primary Attack causes enemy units to attempt to Counter (even if it fails).
Using a Primary Attack will cause Fatigue to your unit, which is a reduction in Efficiency.
Primary Attacks are affected by Attack Power Boosters/Diminishers, Accuracy Boosters/Diminishers, and Efficiency Boosters/Diminishers. All spell/item effects that trigger on an attack type will only modify the primary attack, even if triggered by the extra attack type. [Example: Mind Rippers have a Magic Ranged Primary and a Psychic Ranged Extra Attack, Monkey Brains triggers because they have psychic in the extra attack - but instead will only give the +100% Attack Power bonus to the Primary Attack]].