To decrease means to become smaller, fewer, or less in amount, size, or intensity. When the temperature decreases throughout the evening, it gets colder. When your savings account decreases, you have less money in it than before. When a recipe calls for you to decrease the heat, you turn down the burner to make it cooler.

The word works in both directions: something can decrease on its own (like daylight decreases as winter approaches), or you can actively decrease something (like decreasing the volume on your headphones). You might notice your energy decrease as the school day goes on, or a teacher might decrease the amount of homework assigned over a holiday week.

A decrease (as a noun) is the amount something goes down: “There was a sharp decrease in cafeteria noise after the principal walked in.” Scientists track decreases in animal populations, doctors monitor decreases in symptoms, and coaches watch for decreases in their team's performance.

The opposite of decrease is increase, meaning to grow larger or more. Together, these words help us describe nearly every change we observe: temperatures increase and decrease, prices go up and down, and even your interest in a book might increase or decrease as you read.