A morpheme added at the end of a word to modify the word's meaning.
"The suffix -ness changed the adjective quiet into the noun quietness by indicating a state or quality."
To append (something) to the end of something else.
"The company decided to suffix its new marketing campaign directly after the holiday season sale."
The word "suffix" comes from the Latin suffīxus, meaning "fastened underneath." It is formed by combining sub- ("under") with a form of fīgere ("to fasten").