Blend Mode
A rule determining how a layer's pixels combine mathematically with those beneath it.
In depth
The modes fall into families: multiply and its relatives darken, screen and its relatives lighten, overlay and soft light increase contrast around a mid-grey pivot, and the colour modes separate hue from luminosity. Knowing that a fifty per cent grey layer is invisible in overlay is what makes non-destructive dodging and burning possible.
Examples in practice
Setting a duplicate layer to multiply darkens the image exactly as sandwiching two identical negatives would.