(noun)
a particular stage in a person's development or psychological growth, as during childhood or adolescence, especially when characterized by difficulty or unhappiness;
e.g. He is going through a phase where he feels as though no-one understands him.
(noun)
a distinct part of a recurring cycle, especially the particular shape that the moon or a planet appears to be based on how much of it is illuminated;
e.g. The students were asked to record the phase of the moon each night.
(noun)
(chemistry) a distinct state of matter (solid, liquid, or gas) that remains separate from other states in a system containing multiple states of matter;
e.g. This substance remains in the liquid phase even at very high temperatures.
(verb)
to do or conduct something in a planned and gradual manner, consisting of stages;
e.g. They plan to phase the renovations over the course of many years.
(verb)
to gradually bring something into use or take it out of use, in stages;
e.g. She plans to phase out the old products and bring in the new ones over time.