The term "action" is not explicitly defined by the game rules, but generally refers to the things players can choose to do on their Turns, but it can also refer to things the enemies do.
Typically, choosing an action leads to a procedure which may contain multiple steps. The individual steps in such a procedure are not generally called "actions". However, some actions can lead to a kind of minigame in which actions are taken, usually involving involve a separate gameboard, such as the Port Board used when players Visit a Port, the Island Board used in Exhaustive Expeditions, or the Enemy Ship Board used when [Boarding an enemy ship. Another such minigame is Combat with On Deck enemies, referred to as a Fray, which takes place on the On Deck area on a player's Ship Board.