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.
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British Dictionary definitions for square wave
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
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012