Aspect is a grammatical category that expresses how an action, event, or state, denoted by a verb, extends over time. Perfective aspect is used in referring to an event conceived as bounded and unitary, without reference to any flow of time during ("I helped him"). Imperfective aspect is used for situations conceived as existing continuously or repetitively as time flows ("I was helping him"; "I used to help people").
Certain aspectual distinctions express a relation in time between the event and the time of reference. This is the case with the perfect aspect, which indicates that an event occurred prior to (but has continuing relevance at) the time of reference: "I have eaten"; "I had eaten"; "I will have eaten".
Different languages make different grammatical aspectual distinctions; some (such as Standard German; see below) do not make any. The marking of aspect is often conflated with the marking of tense and mood (see tense–aspect–mood). Aspectual distinctions may be restricted to certain tenses: in Latin and the Romance languages, for example, the perfective–imperfective distinction is marked in the past tense, by the division between preterites and imperfects. Explicit consideration of aspect as a category first arose out of study of the Slavic languages; here verbs often occur in pairs, with two related verbs being used respectively for imperfective and perfective meanings.
The concept of grammatical aspect should not be confused with perfect and imperfect verb forms; the meanings of the latter terms are somewhat different, and in some languages, the common names used for verb forms may not follow the actual aspects precisely.
The Indian linguist Yaska (ca. 7th century BCE) dealt with grammatical aspect, distinguishing actions that are processes (bhāva), from those where the action is considered as a completed whole (mūrta). This is the key distinction between the imperfective and perfective. Yaska also applied this distinction to a verb versus an action nominal.
Grammarians of the Greek and Latin languages also showed an interest in aspect, but the idea did not enter into the modern Western grammatical tradition until the 19th century via the study of the grammar of the Slavic languages. The earliest use of the term recorded in the Oxford English Dictionary dates from 1853.