An attack can be performed in combat, and consists of either unit using their weapon to attempt to damage the other unit.
Attack is a keyword often referred to in skill descriptions, and both initial attacks, follow-up attacks, and counterattacks are considered attacks. Staff users using their assist skill on allies does not constitute an attack, neither does using a 
Dance skill on an allied unit.
A unit can take part in combat without performing any attacks, for example if they are attacked by an enemy while under the influence of 
Counterattacks disrupted, or if their weapon range prohibits them from attacking the other unit.