1. (physical chemistry) a distinct state of matter in a system; matter that is identical in chemical composition and physical state and separated from other material by the phase boundary;
"the reaction occurs in the liquid phase of the system"
2. any distinct time period in a sequence of events;
"we are in a transitional stage in which many former ideas must be revised or rejected"
3. a particular point in the time of a cycle; measured from some arbitrary zero and expressed as an angle
4. (astronomy) the particular appearance of a body's state of illumination (especially one of the recurring shapes of the part of Earth's moon that is illuminated by the sun);