In Vedic timekeeping, a tithi is a "duration of two faces of moon that is observed from earth", known as milа̄lyа̄ (Newar: 𑐩𑐶𑐮𑐵𑐮𑑂𑐫𑐵𑑅, मिलाल्याः) in Nepal Bhasa,^[1] or the time it takes for the longitudinal angle between the Moon and the Sun to increase by 12°. In other words, a tithi is a time duration between the consecutive epochs that correspond to when the longitudinal angle between the Sun and the Moon is an integer multiple of 12°. Tithis begin at varying times of day and vary in duration approximately from 19 to 26 hours.^[2] Every day of a lunar month is called tithi.

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A Hindu muhurta (forty-eight minute duration) can be represented in five attributes of Hindu astronomy namely, vara the weekday, tithi, nakshatra the Moon's asterism, yoga the angular relationship between Sun and Moon and karana half of tithi.

Tithi plays an important role along with nakshatra in Hindus' daily as well as special activities in selecting the muhurta. There are auspicious tithis as well as inauspicious tithis, each considered more propitious for some purposes than for other.

There are 30 tithis in each lunar month, named as:

Tithi is one of the five elements of a Pañcāṅga. The other four elements:

• Nakshatra
• Karaṇa
• Nityayoga
• Vāra