An affix is a letter or group of letters attached to the beginning or end of a word to change its meaning. When you add un- to happy, you create unhappy. When you add -ness to kind, you get kindness. These additions are affixes.

Affixes that go at the beginning of words are called prefixes (like re-, pre-, or dis-), while those that go at the end are called suffixes (like -ful, -less, or -ly). There's even a rare type called an infix that goes in the middle of a word, though English doesn't use these much.

Understanding affixes helps you decode unfamiliar words. When you see unbelievable, you can break it down: un- means “not,” believe is the root word, and -able means “capable of.” Put it together and you get “not capable of being believed.” Once you recognize common affixes, you can figure out hundreds of new words without looking them up. Scientists and writers often create new words by affixing different parts together, which is how English grows and adapts over time.