Grammatical aspect is the grammaticalized expression of the temporal structure of an action or state. Temporal structure roughly relates to duration.
In the English language, verbs express grammatical aspect.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.
Leech, Geoffrey N. 2004. Meaning and the English verb. Harlow, English: Pearson Longman.