seal vs cachet

seal and cachet both are nouns.

Word	Noun	Adjective	Verb	Adverb
seal	Yes	No	Yes	No
cachet	Yes	No	No	No

As nouns, cachet is a hyponym of seal; that is, cachet is a word with a more specific, narrower meaning than seal:

seal: a device incised to make an impression; used to secure a closing or to authenticate documents
cachet: a seal on a letter

Other hyponyms of seal include bulla, great seal, handstamp, rubber stamp, signet.

As nouns, seal and cachet are synonyms defined as:

Other synonyms of seal include seal of approval.

seal (noun)	cachet (noun)
any of numerous marine mammals that come on shore to breed; chiefly of cold regions	a seal on a letter
a device incised to make an impression; used to secure a closing or to authenticate documents	an indication of approved or superior status
fastener consisting of a resinous composition that is plastic when warm; used for sealing documents and parcels and letters	a warrant formerly issued by a French king who could warrant imprisonment or death in a signed letter under his seal
fastener that provides a tight and perfect closure
a finishing coat applied to exclude moisture
an indication of approved or superior status
a stamp affixed to a document (as to attest to its authenticity or to seal it)
the pelt or fur (especially the underfur) of a seal

seal (verb)	cachet (verb)
decide irrevocably
hunt seals
close with or as if with a seal
affix a seal to
make tight; secure against leakage
cover with varnish