Two or more waves traveling in the same direction/arriving at the same point in space, the net sum of which is the sum of each individual wave
-linearity
-phasor diagrams
-ie. plane waves
Interference
In reality, the summation of waves departures from linearity (it is nonlinear at higher amplitudes) and the summation of waves in reality is called interference
Temporal coherence
The crest-to-cret distance stays the same in a single wave
(monochromaticity)
Spatial coherence
All the troughs and peaks line up for multiple waves in motion (minimizing the amount of a source ie. pinhole increases the spatial coherence)