In the video game world a Clan Battle is a pre-arranged matchup between two competing clans.
A battle is agreed upon beforehand; its formal and has specific rules. Often has seperate rooms and passwords on the lobby to keep things tight. Random people are not allowed. Period. And there can be no chance of random people joining.
Unlike scrimmages or clan matches, clan battles are a serious matchup.
For Example: The Old Man Clan OMC sends out a a clan battle challenge for Friday, January 5 at 9:00pm and they challenge the United States Marines USM Clan.
The 2 sides fight in usually a best out of 5 or 7 games in a game mode called Team Death Match. With the winner being the side that meets the pre-determined kill count first.