a graph, function, vibration, etc., that is periodic and equal to one constant on one half of the period and a different constant, which may be zero, on the other half.
square wave voltage waveforms begin to look more sinusoidal due to finite bandwidth.
British Dictionary definitions for square wave
square wave
noun
1.
an oscillation, for example in voltage pulse, that alternates between two fixed values with a negligible transition time between the two, giving a rectangular waveform