Real Dictionary

suffix
countable noun: A suffix is a letter or group of letters, for example '-ly' or '- ness', which is added to the end of a word in order to form a different word, often of a different word class. For example, the suffix '-ly' is added to ' quick' to form 'quickly'. Compare
countable noun: A suffix is one or more numbers or letters added to the end of a code number to indicate, for example, what area something belongs to.

suffix in American English
a letter, syllable, or group of syllables added at the end of a word or word base to change its meaning, give it grammatical function, or form a new word (Ex.: -ish in smallish, -ed in walked, -ness in darkness)
intransitive verb: to admit a suffix
noun: an affix that follows the element to which it is added, as -ly in kindly
transitive verb: to add as a suffix

suffix in British English
sub-
sufficient
abbreviation: suffix
noun: an affix that follows the stem to which it is attached, as for example -s and -ness in dogs and softness
verb: to add (a morpheme) as a suffix to the end of a word