A hostile Action is considered an attack and permits the target or targets to play Reactions against it (unless otherwise specified). An Action is considered hostile if it contains an opposed roll entry, deals damage, imposes conditions or other harmful effects, or otherwise directly harms one or more enemies. Some Actions may explicitly state that they are hostile, particularly if they do not seem to meet one of the prior conditions, but the intent is still to permit a Reaction to take effect. Likewise, some Actions may explicitly state that they are non-hostile even if they seem to meet one of the prior conditions when the intent is to prevent enemies from playing any Reactions.
The terms “attack” and “hostile Action” are equivalent. The “attack” term is most frequently used in combat scenarios, while “hostile Action” is often preferred in interaction scenarios. This is simply a distinction of flavor and theme, not of mechanical function.