Definition of red letter in US English:

red letter

noun

  • 1A character or word written or printed in red, typically in order to draw attention to particularly important or significant information. Chiefly in plural.

  • 2A small moth, Agonopterix ocellana (family Oecophoridae), of Eurasia and North Africa, having a white spot ringed with red in the centre of the pale forewing. Also "red-letter flat-body". rare.

Origin

Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator. From red + letter.

Pronunciation

red letter

/ˌrɛd ˈlɛtə/