Alt is an abbreviation for alternate character. Virtually all MMORPGs permit a player to create more than one player character on his or her account. An alt is a character to which a person devotes less time and effort than the person's "main" - their primary character.
What constitutes "less" time and effort can be subjective with respect to the player. For most players, the main is the highest-level, most powerful, or simply the oldest character on the account, while some refer to all of their characters on their main server as "mains" and their characters on other servers as alts. Some players play multiple characters at around the same level, keeping them roughly equal in power. In those cases, players often simply use the term alt to refer to a character other than the one they are currently playing.
Alts are created for a number of reasons.
Players often create alts to have different gameplay, roleplay, or social experiences. They may be created to alleviate boredom, or to try something new after reaching a milestone like the level cap. World of Warcraft includes eleven classes, thirteen races in two factions, eleven primary professions, and PvP- and non-PvP realm types to provide many gameplay options which can only be experienced with alts (or paid character modification services). These choices are equally meaningful in roleplaying, which may itself vary between alts on Roleplay or non-Roleplay realm types. Roleplayers may also develop different names, personalities and backstories, and utilize different roleplaying gear between characters.
World of Warcraft does not reveal which characters belong to the same player by default, which allows players to choose who they would like to make aware of different alts. This allows alts to be more or less involved in guild activities, have smaller, larger, or completely different groups of friends, or even be completely socially isolated. This choice is diminished somewhat if players choose to become Real ID or BattleTag friends with others, since those friends will be able to see any character being played, but players are free to use those features as widely or narrowly as they like - and, of course, those friends may be more likely to respect players' wishes with regard to use of alts.
Reinforcing other characters
Some alts are created on the same realm to increase the power of a player's main, or to create a team of characters all mutually reinforcing each other. Many quests and recipes in the game require materials not available to single characters without help. For instance, a tailoring recipe may require ingredients obtained only by skinning or mining—or both—while a single character may possess only two of those types of professions at a time. With alts, a player can work other professions, and avoid problems with availability of materials, at the expense of spending extra time leveling up. A character may even be enabled to learn an unconventional mix of crafting professions if an alt knows two gathering professions. For instance engineering is normally taken with mining and inscription with herbalism to avoid relying on other players for materials, but engineer/scribes are feasible if miner/herbalist alts exist to supply them.
Alts created primarily for gathering resources for other characters are commonly known as farming alts. Death Knights are popular for this purpose since they begin the game at level 55 and therefore have a head start at gaining access to all the areas in the game where resources can be found. Farming alts may be used to supply other characters of the same player, or simply to gather materials to auction and make gold.
A bank alt or "mule" is an alternate character that is usually level 1 and sits in a trade city. This character's primary use is for:
- Extra bank space, possibly by being the sole member of a 1-member guild with a guild bank
- Auction and chat sales convenience by remaining next to the Auction House, allowing the main to mail items to sell instead of traveling
- Management of numerous amounts of resources via the mail, bags, and bank, isolating the main's storage for cleaner organization
Some alts exist primarily:
- to be twinks (to excel in battleground brackets below the level cap)
- to aid in debugging addons that would put "real" characters at risk of losing gold and/or items
These types of characters are not always played in the normal sense; see their respective pages for more information.
Functionality on a single account
Most players have a single World of Warcraft account, which allows up to 50 characters to be created at a time across all servers.
Alts on the same account, realm and faction can send mail to each other. Items and gold normally take one hour to deliver when mailed to characters outside of the sender's guild (assuming the perk), but mail sent to alts will arrive instantly, allowing for easy transfer of crafted, collected, or purchased items. Bind to Account items such as heirlooms have even more freedom, and can be mailed to any alt on any faction and realm.
Alts on the same realm and faction can be in the same guild and use the same guild bank. There is no account-based restriction on using the guild bank to transfer items or gold, although different alts may have different guild ranks with varying bank privileges.
Alts on the same account cannot be logged into the game world simultaneously. This prevents such alts from directly supporting each other in combat (known as multiboxing) and from transferring items directly using the trade window. Limited-duration items can only be given to other characters using the trade window, and consequently cannot be passed directly to an alt. Alts also cannot buy each other's auctions, preventing transfers between Horde and Alliance alts on the same account using the auction house.
Use of multiple accounts
There is no restriction on the number of accounts you can have, but each costs an account fee per month. Alts on separate accounts circumvent the item transfer restrictions placed on alts on the same account, and can still send and receive Bind to Account items as long as all the accounts are registered under the same Battle.net account. However, few players pay for a separate account just to occasionally trade items; the primary use of alts on separate accounts is multiboxing.
You could run an alt on a trial account, but that trial account would not be able to trade, use the auction house, nor mail or receive items, so a trial account alt cannot get around the trading restrictions of a same account alt.
You can multibox with trial account alts, but gearing your alts is a problem, since you cannot directly share. The best you can do is run as master looter on your main to allocate items that drop. You will also quickly exceed the level limit for trial accounts. Anyone serious about multiboxing will probably prefer to run paid accounts.
Players who have a large number of mid-level characters rather than a few high-level ones are sometimes jokingly accused of suffering from "alt-itis". Symptoms include:
- Playing for over a year without ever getting a mount
- Trouble keeping mules — you just can't stop leveling them!
- Running out of global character slots (a player can only have 50 characters across all realms, and 10 on each individual realm)
- Needing an addon to keep your Friends lists synched
- Inability to commit any characters to max level or a even a twink level
This term is almost always used in good humor, despite the usual implication that it's a sign of indecision. The most common causes of alt-itis are:
- Wanting to experience as much of the game as possible (different classes, races, professions, zones, race-/class-/faction-specific quests)
- Trying to socialize with as many people as possible (with enough online friends, you seem to end up with one at every level)
- Before paid character transfers, needing to start over on a new server
- Enjoying lower level content more than higher level content, especially the more solo-friendly quests and equipment progression before level cap
- Addiction to the fast level-up rate of levels 1 through 20
- Creating multiple characters on roleplaying servers, each with different backgrounds and histories
- Various mental organization disorders such as Attention Deficit Hyperactivity Disorder  or Obsessive-Compulsive Disorder 
Many sufferers of alt-itis are proud to wear the label.
Alternate terms include:
- Alt-aholic (World of Warcraft can be a "drug" in a modern brain chemistry sense)