Science

State of matter, or phase, one of the distinct forms in which matter can exist
Phase (matter), a region of space throughout which all physical properties are essentially uniform
Phase space, a mathematical space in which each possible state of a physical system is represented by a point — this equilibrium point is also referred to as a "microscopic state"
- Phase space formulation, a formulation of quantum mechanics in phase space
Phase (waves), the position of a point in time (an instant) on a waveform cycle
- Instantaneous phase, generalization for both cyclic and non-cyclic phenomena
AC phase, the phase offset between alternating current electric power in multiple conducting wires
- Single-phase electric power, distribution of AC electric power in a system where the voltages of the supply vary in unison
- Three-phase electric power, a common method of AC electric power generation, transmission, and distribution
Phase problem, the loss of information (the phase) from a physical measurement
Phase factor, a complex scalar used in quantum mechanics
In continuous Fourier transform, the angle of a complex coefficient representing the phase of one sinusoidal component
The argument or angle of complex numbers is sometimes called the phase

Archaeological phase, a discrete period of occupation at an archaeological site
Color phase, in biology, a group of individuals within a species with a particular coloration
Gametic phase, in genetics, the relationship between alleles at two chromosomal loci
Lunar phase, the appearance of the Moon as viewed from the Earth
Planetary phase, the appearance of the illuminated section of a planet
Phase separation, in physical chemistry, the separation of a liquid mixture into two immiscible liquids above and below a meniscus
Phase (syntax), in linguistics, a cyclic domain (proposed by Noam Chomsky)
In biology, a part of the cell cycle in which cells divide (mitosis) and/or reproduce (meiosis)

Music

Phase (band), a Greek alternative rock band
Phases (band), an indie pop American band, formerly known as JJAMZ
Phase (Jack Garratt album), 2016
Phases (The Who album), a 1981 box set of albums by The Who
Phases (I See Stars album), 2015
Phase (Mildlife album), 2017
Phases (Angel Olsen album), 2017
"Phases", a song by English alternative rock band Keane from their 2019 album Cause and Effect

"Phases" (Buffy the Vampire Slayer), a 1998 episode of the TV series Buffy the Vampire Slayer
Phases (.hack), fictional monsters in the .hack franchise
Phase, an incarnation of the DC Comics character usually known as Phantom Girl

Phase 10, a card game created by Fundex Games
Phase (video game), a 2007 music game for the iPod developed by Harmonix Music Systems
Phase (combat), usually a period of combat within a larger military operation
A musical composition using Steve Reich's phasing technique