Panchanga · तिथिः
Tithi — The Lunar Day
A Tithi is the time for the Moon's angular distance from the Sun to increase by exactly 12°. Thirty Tithis make one lunar month, split into two 15-day Paksha (fortnights).
Shukla Paksha (waxing) — 15 Tithis
Krishna Paksha (waning) — 15 Tithis
Why Tithi length varies
Because the Moon's orbital speed isn't constant, a Tithi can run anywhere from about 19 to 26 hours — never exactly 24. This is why, on rare occasions, a Tithi is skipped within a solar day (Kshaya Tithi) or spans across two (Vriddhi Tithi).
See Paksha for more on the two fortnights, or the wider Panchanga overview.