To deteriorate means to gradually become worse or break down over time. When something deteriorates, it loses quality, strength, or usefulness bit by bit.

Think of a banana sitting on the counter: at first it's perfectly yellow and firm, but day by day it deteriorates until it's brown, mushy, and eventually inedible. Old buildings deteriorate when their paint peels, their walls crack, and their foundations weaken. A friendship can deteriorate if two people stop talking honestly with each other, letting small misunderstandings pile up into bigger problems.

The word captures that slow, steady decline rather than sudden destruction. A book doesn't deteriorate if you accidentally drop it once, but it will deteriorate if you leave it outside in the rain and sun for months. Your math skills might deteriorate over summer break if you don't practice.

Deterioration is the noun form: “The deterioration of the old bridge forced the city to close it.” The opposite of deteriorate is improve or strengthen. When doctors say a patient's condition is deteriorating, they mean the person is getting sicker rather than better.