Definition: The emotions that occur as a direct result of encountering some kind of cue. For example, if someone is late for a meeting that is scheduled, you may experience frustration or concern. These emotions would be considered a primary emotion, because the emotion occurred as a direct consequence of encountering some kind of event.
Primary emotions are "fast-acting." That is, they occur in close proximity to the event that brought them on. Primary emotions are important, because they provide us with information about our current situation and get us ready or motivated to act in some way.