An alternative account is the account of a person who already has at least one, usually for the purpose of avoiding or protesting a ban. The most prolific users of alternative accounts are allegedly Ferrets54 and Atheist Peace.
The ToS was later altered to make it okay for users to have more than one account on the forums. However, each individual may only have one that is the rank of citizen and any infractions earned by one account are attributed to all that user's accounts. Often people will create alt accounts for comedic value, stereotyping and for simple entertainment.