A divinely commanded law in the Torah. In the singular it is mitzvah, in the plural mitzvot. According to the aggadah there are 613 mitzvot in the Torah, 248 positive mitzvot ("Thou shalt...!"), and 365 negative ("Thou shalt not...!"). There is a positive mitzvot for each bone in the human body -- according to the counting of Aristotle(*) -- to tell us that every limb of the body cries out, "Perform a mitzvah with me!". There is a negative mitzvah for every day in the year, to teach us that every day cries out, "Please do not sin on me!".
(*) Modern anatomists identify more than this.