The waning gibbous covers elongation from a hundred and eighty degrees to two hundred and seventy, between full moon and last quarter, with more than half the disc lit and the illuminated portion shrinking. It is the mirror of the waxing gibbous in appearance and its opposite in timing: the Moon now rises after sunset and is highest in the small hours.
The phases are produced by the changing angle between the Sun, the Earth and the Moon, and the quantity that defines them is elongation, the Moon's angular distance from the Sun as seen from the Earth. The complete round from one new moon to the next is the synodic month, about 29.53 days, and it is longer than the sidereal month of about 27.32 days in which the Moon returns to the same place among the stars, because the Earth has moved along its orbit in the meantime and the Moon must travel further to catch up.
The waxing and waning halves of the cycle are geometrically symmetrical and culturally are not. A very large number of traditions treat the growing Moon and the diminishing Moon as carrying different qualities, and the Panchanga formalises the division: the bright fortnight, shukla paksha, runs from new to full, and the dark fortnight, krishna paksha, from full back to new, each of fifteen tithis, and the same tithi name occurs once in each with a different character.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Indian CalendarRobert Sewell and Sankara Balkrishna Dikshit