An athlete is a person who has above average physical skills (strength, agility, and endurance) and is thus suitable for physical activities, in particular, contests. An ancient Greek word for "contest" was athlos, and those competing in the games were called athletes.
In more specific settings, an athlete is one who participates in competitive events such as professional sports. In British English this is almost exclusively limited to athletics.
The word can also, although less often, refer to a mental, instead of physical, competitor or possessor of skills. Athletes can play many sports and they're not all the same shape and size.