A waveform that rises quickly to a particular amplitude, remains constant for a time period and drops fast at the end. In digital systems, square waves are the norm, because they represent a binary digit (0 or 1). Square waves can also be generated in musical synthesizers and have a raspy sound.
This is the output of a 1 kHz square wave on an oscilloscope. Only the tops and bottoms of the waves are displayed, because the rising and falling is faster than the scope is set for.