cachet vs lettre de cachet

cachet and lettre de cachet both are nouns.

Word	Noun	Adjective	Verb	Adverb
cachet	Yes	No	No	No
lettre de cachet	Yes	No	No	No

As nouns, cachet and lettre de cachet are synonyms defined as:

cachet and lettre de cachet: a warrant formerly issued by a French king who could warrant imprisonment or death in a signed letter under his seal

cachet (noun)	lettre de cachet (noun)
a seal on a letter	a warrant formerly issued by a French king who could warrant imprisonment or death in a signed letter under his seal
an indication of approved or superior status
a warrant formerly issued by a French king who could warrant imprisonment or death in a signed letter under his seal