Nakshatra
One of 27 star-segments the Moon passes through.
The ecliptic divided into 27 equal segments of 13°20', each named for a star or asterism. The Moon passes through roughly one a day. Several festivals are set by nakshatra rather than tithi — Karthigai Deepam and Onam among them — which is why they can fall on a different day from the tithi-based calendar's expectation.