A phase is a distinct stage or period in a process of change or development. When something happens in phases, it unfolds step by step rather than all at once. A construction project might be completed in three phases: first the foundation, then the frame, and finally the interior work.

Scientists use phase to describe different states of matter: ice, water, and steam are three phases of H₂O. The moon goes through phases as we see different amounts of its lit surface from Earth: new moon, crescent, half moon, and full moon follow each other in a predictable cycle.

People also use phase to describe temporary behaviors or interests, especially in young people. If your little brother suddenly refuses to eat anything green, your parents might say “it's just a phase,” meaning they expect it to pass. When someone says they're “going through a phase,” they usually mean a period of trying something new or acting differently than usual.

The verb form means to introduce gradually: a school might phase in a new reading program by starting with one grade level at a time, or phase out an old policy by slowly replacing it with something new.