Pūrṇimā (Sanskrit: पूर्णिमा) is the word for full moon in Sanskrit.
The day of Purnima is the day (Tithi) in each month when the full moon occurs, and marks the division in each month between the two lunar fortnights (paksha), and the Moon is aligned exactly in a straight line, called a syzygy, with the Sun and Earth.
The full moon shows 100% illumination, causes high tides, and can concur with lunar eclipses.
[1] The following festivals occur on the purnima.
The Manava Purana (one of the Upapuranas) contains a list of the festivals that fall on the full moon.