Phase (Wave)
Definition
A wave's position within its cycle — telling you whether it is at a crest, trough, or somewhere in between.
Symbol: · Unit: radians (or degrees)
Technical definition
The argument of the sinusoidal wave function, φ = kx − ωt + φ₀, measured in radians or degrees. Two waves are ‘in phase’ when their phase difference is a multiple of 2π (constructive interference); ‘out of phase’ by π gives destructive interference.
Example
Two speakers playing the same tone are loudest where their waves arrive in phase (crests align) and can cancel to near-silence where they arrive exactly out of phase.
Common confusion
Phase is not the same as phase difference. A single wave's absolute phase is arbitrary — what matters physically is the phase difference between waves, which determines interference.