English
lightlike
From light + -like.
Formed from light + -like
Proto-Indo-European *lewk- ‘to shine’
…
Proto-Germanic *leuhtą
Proto-West Germanic *leuht ‘light’
Old English lēoht
Middle English light ‘light, illumination’
English light
Today it means
adj (of a four-vector) Having a space component whose magnitude is equal to its time component multiplied by the speed of light.
Language periods are conventional ranges, not dates for this word.
Etymograph / lightlike