When I was three, I went through a phase when I had an imaginary friend.
a period of time when someone exhibits unusual behavior — most commonly said of a young person who is going through a stage such as a girl who doesn’t want different types of food to touch on her plate
The increase in electricity rates will be phased in over the next two years.
make a change gradually (... in =gradually added; ... out = gradually removed or stopped)
More specialized and less common senses of the word are found in astronomy, physics, chemistry, and zoology. Consult a comprehensive dictionary if you wish to see those. All senses have to do with stages or periods of time.