Wiktionary
triangle wave
n. (context maths engineering English) A function or waveform that alternately ramps linearly upwards and then downwards.
Wikipedia
Triangle wave
A triangle wave is a non-sinusoidal waveform named for its triangular shape. It is a periodic, piecewise linear, continuous real function.
Like a square wave, the triangle wave contains only odd harmonics, demonstrating odd symmetry. However, the higher harmonics roll off much faster than in a square wave (proportional to the inverse square of the harmonic number as opposed to just the inverse).