A declared width at which a layout changes shape. The point where a design admits the space has changed.
Compound of break (Old English brecan) and point (Old French point, from Latin punctum, a prick, a dot); a breakpoint names the exact spot where a continuous thing is declared to change.
A declared width, or a declared line, at which the thing built is allowed to change shape — the point where a design admits the space it is in has changed.