A distinct stage of development: "The American occupation of Japan fell into three successive phases\u201d ( Edwin O. Reischauer). A temporary manner, attitude, or pattern of behavior: just a passing phase. An aspect; a part: every phase of the operation. Astronomy One of the cyclically recurring apparent forms of the moon or a planet. Physics A particular stage in a periodic process or phenomenon. Physics The fraction of a complete cycle elapsed as measured from a specified reference point and often expressed as an angle. Chemistry Any of the forms or states, solid, liquid, gas, or plasma, in which matter can exist, depending on temperature and pressure. Chemistry A discrete homogeneous part of a material system that is mechanically separable from the rest, as is ice from water. Biology A characteristic form, appearance, or stage of development that occurs in a cycle or that distinguishes some individuals of a group: the white color phase of a weasel; the swarming phase of locusts. To plan or carry out systematically by phases. To set or regulate so as to be synchronized. phase in To introduce, one stage at a time. phase out To bring or come to an end, one stage at a time. in phase In a correlated or synchronized way. out of phase In an unsynchronized or uncorrelated way.