means A distinct stage or period within a larger process of change, or a state in which something exists, such as solid, liquid, or gas.

from From Greek 'phasis,' meaning 'appearance' or 'a showing forth' — rooted in 'phainein,' 'to show, bring to light' (the same source that lights up 'phenomenon' and 'fantasy'). Astronomers first borrowed it for the changing faces of the Moon as it waxed and waned across the sky; from those lunar appearances it spread to any recurring stage in a cycle, and later into physics for the states of matter.