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Viewpoint aspect (perfective vs. imperfective) is a grammatical category marking how the speaker presents the temporal contours of an event. Perfective aspect views the event as a bounded whole; imperfective aspect views it as internally structured or ongoing. This is orthogonal to lexical aspect (aktionsart) and is typically marked morphosyntactically.
From your study of tense-aspect semantics and event semantics, you know that events have temporal structure — they unfold across intervals, have culmination points, and can be described at different levels of granularity. From Aktionsart (lexical aspect), you know that the verb itself encodes whether an event is inherently bounded (*accomplish*), unbounded (*run*), instantaneous (*sneeze*), or stative (*know*). Viewpoint aspect is the second dimension of aspect, and it is grammatical rather than lexical: it reflects a *choice the speaker makes* about how to present the temporal contours of an event, regardless of what the verb's Aktionsart is.
The perfective viewpoint presents an event as a bounded, completed whole — the speaker views the event from outside, as if seeing it in its entirety from beginning to end. In Russian, *ya napisal pis'mo* ("I wrote the letter," perfective) presents the letter-writing as a completed unit with a clear endpoint achieved. The perfective does not tell you how long the event took; it tells you that the speaker is treating it as a closed, totalized event. In English, the simple past often has perfective implications, though English is not a grammatically aspectual language in the way Slavic languages are.
The imperfective viewpoint presents the event as internally open — the speaker zooms in on its interior, presenting it as ongoing, habitual, or without reference to its endpoint. *Ya pisal pis'mo* ("I was writing the letter," imperfective) presents the writing as an activity in progress, without asserting it was completed. Crucially, the same event can be described with either viewpoint: the letter may or may not have been finished in both cases. The aspectual choice reflects a perspectival decision, not a factual one about whether completion occurred.
The key theoretical point — that viewpoint aspect is orthogonal to lexical aspect — means you can apply either viewpoint to almost any verb. An achievement verb like *notice* is lexically instantaneous, yet you can impose imperfective viewpoint on it: "I was just noticing how quiet it had become" uses an imperfective frame on an achievement. A state verb like *know* is lexically atelic, yet a perfective can be imposed: "She knew the answer at that moment" presents a bounded slice of the state. This orthogonality is the diagnostic criterion for separating the two dimensions: if you can vary aspect independently of the verb's inherent Aktionsart, you have evidence for a grammatically encoded viewpoint system rather than a purely lexical one.