An alternate account is an additional account created by an existing forumgoer, usually to obscure their identity. Sometimes players will create new accounts to avoid metagaming based on their previous behavior, or to get away from indignities/personality conflicts. This is allowed (if not encouraged) on MafiaScum.net, provided the player does not try to use the two accounts to each others' advantage (such as playing in the same game, fishing for information with one account about something the other account is doing/playing, or otherwise circumventing the rules).
Anonymafia games are games that are played entirely with alternate accounts.
It's possible that players with an alternate account fail to post from the correct username in game threads. This is known in the community as an altslip. Historically, altslips have not resulted in punishment, so long as they are "picked up" by admitting on your main account that you made a mistake in doing so.
Rules regarding alt slips tend to be clearly defined in Anonymafia games.
Changing a website theme per account is a popular method of maintaining the sanctity of alt accounts.
See also: Moderator Account.