Fraternity is a society of college or university students and graduates. Fraternities are also called Greek-letter societies because most fraternities form their names by combining two or three letters of the Greek alphabet. The word fraternity comes from the Latin word frater, meaning brother.

The best-known kind of fraternity is the general or social fraternity. Professional fraternities are made up of people who are preparing for or working in such professions as education, law, medicine, and science. Fraternities called honor societies select their members for their exceptional academic records. Recognition societies are for people with superior achievement in a specific undertaking. A student may join only one general fraternity. But a general fraternity member may also join a professional fraternity, an honor society, or both.

Many fraternities admit both men and women, but most general fraternities are for men. Women's organizations for college students are discussed in the Sorority article. Most fraternities have chapters (local units) throughout the United States and Canada. Intercollegiate, national, and international fraternities have thousands of chapters with millions of members.

The first fraternity, Phi Beta Kappa, was founded in the United States in 1776 at the College of William and Mary in Williamsburg, Virginia. It began as a general fraternity, then later became an honor society. The Kappa Alpha Society, founded in 1825 at Union College in Schenectady, New York, is the oldest ongoing general fraternity.