The prefix tri- means three. It comes from Latin and Greek, and you'll find it attached to the beginning of many English words to signal that something involves the number three.

A tricycle has three wheels. A triangle has three sides and three angles. When you read a trilogy of books, you're reading three connected stories. A triathlon challenges athletes to compete in three different sports: swimming, biking, and running.

Sometimes tri- appears in words where the “three” connection isn't immediately obvious. Triple means three times as much. A tripod is a three-legged stand that photographers use to keep cameras steady. Triceratops describes a dinosaur with a three-horned face and a distinctive skull.

When you see tri- at the start of a word, you've got a helpful clue about its meaning. If your teacher mentions a trimester, you know the school year splits into three parts. If someone describes a tricolor flag, you can picture three colors. Recognizing this prefix helps you decode unfamiliar words and understand new concepts more quickly.