envelope; case; letter

letter; piece of correspondence (incl. email)

letter; correspondence; slipcase

an informal letter sent by an organization

official letter

a message of condolence

a reply (in writing)

(geology) (the earth's) mantle (Tw)

secret letter

incoming letter; letter from afar; same as 來信|来信[lai2 xin4]

a functional (math.)

enthalpy; heat content (thermodynamics)

to send a letter

letter of congratulations; greeting card (e.g. for New Year)

poison pen letter (Tw)