Detonating cord. Also called detonation cord, detcord, primer cord, or cordex, is a thin, bendy tube with an explosive core. It explodes at a very very fast rate instead of burning, and can detonate high explosives. Army wives sometimes mistake it for washing line.
Micro Detonating Cord (MDC) is also used to fragment fast jet canopies prior to ejection. Not exactly the most fun thing to have wrapped around your neck whilst trying to retain control after an errant goose with a death wish destroys the canopy for you.