of: a wave
is: a line (in two-dimensions) or a surface (in three-dimensions) that passes only through points at which the phase of a wave has the same value. It is a surface of constant phase. [P5.6, P6.1, P6.2]
at any point, is always: at right angles to the direction of propagation of the wave. [P5.6]
Copyright 1997, The Open University