If a player has been dealt 21 points of combat damage by a particular Commander during the game, that player loses a game. Details

This is an additional state based effect.
Commander Damage is cumulative throughout the game; nothing can reduce the amount of damage a Commander has previously done to a player.
Because it is a property of the card and not a characteristic of the game object, a card is still the same Commander even if it leaves the field and returns.
While effects can raise a player's life total, it doesn't reduce the amount of damage previously taken from a Commander. (eg: Beacon of Immortality)
Conversely, combat damage can be reduced, prevented, or replaced as it is taken, in which case it was never dealt and doesn't count towards the total taken from that Commander. (eg: Fog or Captain's Maneuver)
Commander Damage is specific to each Commander/Player pairing, not combined across all Commanders.
A player can lose if he or she is dealt 21 points of combat damage by his or her own Commander (ie: under someone else's control).