Definition of light speed in US English:

light speed

noun

  • The speed of light. Also (hyperbolically): an extremely high speed.

adjective

  • Of or relating to the speed of light. Also (hyperbolically): extremely fast.

Origin

Early 20th century; earliest use found in The Frederick Post. From light + speed.

Pronunciation

light speed

/ˈlʌɪt ˌspiːd/