Aspect (or grammatical aspect) refers to grammatical marking of the internal temporal structure of an event (beginning, middle and end).
The imperfective aspect refers to a moment within the boundaries (beginning, end) of an event, or, put differently, the event has begun before reference time and continues after reference time. In narrative texts, imperfective markers are often used to refer to backgrounded events.
The perfective aspect refers to the whole situation, with beginning, middle and end. In narrative texts, perfective markers are often used to refer to foregrounded events (story-line events). In many creole languages, especially in the Atlantic creoles, it is the zero-marked verb which fulfills the function of the perfective aspect.
Some authors like Derek Bickerton refer to the imperfective aspect as "durative" and to the perfective aspect as "punctual", but these two labels fit better for lexical aspect (or aktionsart) than for grammatical aspect.
The definition of aspect used in APiCS is a narrow definition of aspect; there are other aspectual categories like resultative, completive, inchoative, etc. If we speak of aspectual or temporal-aspectual systems, as in Feature 49, we consider only the presence (or absence) of imperfective and perfective aspect.