Overview
In Vedic timekeeping, a tithi is a lunar day, defined as the duration during which the longitudinal angle between the Moon and the Sun increases by 12°. In other words, a tithi is the time interval between consecutive moments when the angular separation of the Sun and the Moon is an integer multiple of 12°. Each day of a lunar month in the traditional Hindu calendar is reckoned as a tithi.
Because the Moon's motion relative to the Sun is not uniform, tithis do not have a fixed length. They begin at varying times of day and their duration ranges approximately from 19 to 26 hours. As a result, a tithi may sometimes span parts of two solar days, or, less often, a single solar day may contain the beginning and end of a tithi within it.
The concept is also reflected in regional traditions of the Indian subcontinent and neighbouring regions. In Nepal Bhasa, a tithi is known as milа̄lyа̄ (Newar: 𑐩𑐶𑐮𑐵𑐮𑑂𑐫𑐵𑑅, मिलाल्याः), described as the duration of two faces of the Moon as observed from the Earth. Tithis form the basis for determining the dates of festivals, fasts and other observances in Hindu, Buddhist and related calendrical traditions.
References
Adapted from the English Wikipedia article on Tithi.