define: phase
Definition of phase. 1  1 : a particular appearance or state in a regularly recurring cycle of changes phases of the moon. 2  2a : a distinguishable part in a course, development, or cycle the early phases of her careerb : an aspect or part (as of a problem) under consideration.
1. countable noun. A phase is a particular stage in a process or in the gradual development of something. This autumn, 6000 residents will participate in the first phase of the project. The crisis is entering a crucial, critical phase.