the.com/spark gap

means A small gap between two conductors that electricity leaps across as a spark when the voltage is high enough, used to signal switching, trigger ignition, or detect/generate radio waves.

from Straightforward compound English — 'spark' (a small fiery particle, used since the Middle Ages for flashes of fire or wit, later extended to electrical discharge) plus 'gap' (an opening or breach, from Old Norse 'gap,' meaning chasm or opening). The term became technical vocabulary in the 1800s as scientists like Hertz used actual physical gaps between electrodes to produce and detect electromagnetic waves — the device literally named for what it does: a gap, that sparks.