Muhammad Ali (born Cassius Marcellus Clay Jr.; January 17, 1942) is a retired American (United States) boxer (boxing) and three-time World Heavyweight Champion (List of heavyweight boxing champions), who is widely considered one of the greatest heavyweight championship boxers of all time. As an amateur, he won a gold medal in the light heavyweight division at the 1960 Summer Olympics in Rome. After turning professional, he went on to become the first boxer to win the lineal heavyweight championship (lineal championship) three times.

Originally known as Cassius Clay, Ali changed his name after joining the Nation of Islam in 1964, subsequently converting to Sunni (Sunni Islam) Islam in 1975 and more recently to Sufism. In 1967, Ali refused to be inducted into the U.S. military based on his religious beliefs and opposition to the Vietnam War. He was arrested and found guilty on draft evasion charges, stripped of his boxing title, and his boxing license was suspended. He was not imprisoned, but did not fight again for nearly four years while his appeal worked its way up to the U.S. Supreme Court, where it was successful.

Nicknamed "The Greatest", Ali was involved in several historic boxing matches. Notable among these are three with rival Joe Frazier and one with George Foreman, whom he beat by knockout to win the world heavyweight title for the second time. He suffered only five losses (four decisions and one TKO (Technical knockout) by retirement from the bout) with no draws in his career, while amassing 56 wins (37 knockouts and 19 decisions). Ali was well known for his unorthodox fighting style, which he described as "float like a butterfly, sting like a bee", and employing techniques such as the rope-a-dope. He was also known for his pre-match hype, where he would "trash talk (Trash-talk)" opponents on television and in person some time before the match, often with rhymes. These personality quips and idioms, along with an unorthodox fighting technique, made him a cultural icon. In later life, Ali developed Parkinsons disease. In 1999, Ali was crowned "Sportsman of the Century" by Sports Illustrated and "Sports Personality of the Century" by the BBC.