means a letter or group of letters attached to the front of a word to change its meaning, like 'un-' in 'unhappy' or 're-' in 'rewrite'

from From Latin praefixus, the past participle of praefigere, 'to fasten in front,' built from prae- ('before') plus figere ('to fix or fasten' — the same root that gives us 'fix' and 'fixture'). So a prefix is literally something fastened before — a piece nailed onto the front of a word.