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.
Aspect is often confused with the closely related concept of tense, because they both convey information about time. While tense relates the time of referent to some other time, commonly the speech event, aspect conveys other temporal information, such as duration, completion, or frequency, as it relates to the time of action. Thus tense refers to temporally when while aspect refers to temporally how. Aspect can be said to describe the texture of the time in which a situation occurs, such as a single point of time, a continuous range of time, a sequence of discrete points in time, etc., whereas tense indicates its location in time.
For example, consider the following sentences: "I eat", "I am eating", "I have eaten", and "I have been eating". All are in the present tense, indicated by the present-tense verb of each sentence (eat, am, and have). Yet since they differ in aspect each conveys different information or points of view as to how the action pertains to the present.
Grammatical aspect is a formal property of a language, distinguished through overt inflection, derivational affixes, or independent words that serve as grammatically required markers of those aspects. For example, the K'iche' language spoken in Guatemala has the inflectional prefixes k- and x- to mark incompletive and completive aspect; Mandarin Chinese has the aspect markers -le 了, -zhe 着, zài- 在, and -guò 过 to mark the perfective, durative stative, durative progressive, and experiential aspects, and also marks aspect with adverbs; and English marks the continuous aspect with the verb to be coupled with present participle and the perfect with the verb to have coupled with past participle. Even languages that do not mark aspect morphologically or through auxiliary verbs, however, can convey such distinctions by the use of adverbs or other syntactic constructions.
Grammatical aspect is distinguished from lexical aspect or aktionsart, which is an inherent feature of verbs or verb phrases and is determined by the nature of the situation that the verb describes.