Each of 27 (or, according to some texts, 28) equal divisions of the ecliptic through which the moon passes during the course of a sidereal month; a lunar mansion; the period of time with which each of these corresponds.
The 28th nakshatra of later ritual sources is purely nominal and does not affect the calendar. It probably represents a contamination of the basic astronomical system with lay notions of calendar weeks, months, and solar days.
Late 18th century; earliest use found in Philosophical Transactions of the Royal Society of London. From Sanskrit nakṣatra star, lunar mansion (literally ‘with dominion over night’) from nak, nakt- + kṣatra dominion.