Happening now rather than recorded. Also carrying current — both mean a thing presently in effect.
Old English libban/lifian (to live, be alive), from Proto-Germanic libjan, related to lif (life); happening now rather than recorded, and, of current, a force presently in effect.
Presently in effect, whether the sense is a broadcast happening now or a wire carrying current — both mean the thing is active, not merely stored.