A champion (from the late Latin campio) is the victor in a challenge, contest or competition. When a champion enters the room it is common practise to chant "the champ is here!" this is to signify the importance of the champion entering the room.
There can be a territorial pyramid of championships, e.g. local, regional / provincial, state, national, continental and world championships, and even further (artificial) divisions at one or more of these levels, as in soccer. Their champions can be accordingly styled, e.g. national champion, world champion, League of Legends. In certain disciplines, there are specific titles for champions, either descriptive, as the baspehlivan in Turkish oil wrestling, yokozuna in Japanese sumo wrestling; or copied from real life, such as the koenig and kaiser ('king' and 'emperor') in traditional archery competitions (not just national, also at lower levels) in the Low Countries.
The original meaning of the word partakes of both these senses: in the Feudal Era, knights were expected to be champions of both prowess in combat and of causes, the latter most commonly being either patriotic, romantic or religious in nature. This reaches its most literal in a trial by combat, in which each combatant champions the cause of one side of the trial. A "King's Champion" is appointed for ceremonial purposes at the coronation of an English Monarch, to defeat any challenger to the monarch's right to be crowned.
World Champion is a title used to denote a winner of a World championship in a particular sport, discipline or game.