Alt is an abbreviation for alternate character, or alter-ego. 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 character.
What constitutes "less" time and effort can be subjective with respect to the player. For most players, the main is the most powerful, highest-level, or simply the oldest character on the account while some refer to all of their characters on their main server as "mains" while their characters on other servers are referred to as alts. A few players may play multiple characters simultaneously, 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 variety of reasons.
Some players create alts to have a different social experience. In some cases, the change is minor, being simply a second character with a slightly different personality and/or abilities. In other cases it can be drastic, as in being a member of a different race or gender. Alts may have wholly different circles of friends from the main, or no friends at all.
In WoW, many players create alts to try different classes and professions, to enrich their gameplay, or to remedy boredom. People who bought the game when it first came out typically reached the maximum level of 60 within a few months, and started alts. WoW does well in this regard, by providing ten different races (twelve after the Cataclysm), ten classes, and four realm types; it's easy to encounter a completely different gameplay experience.
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 person can work other professions, and avoid problems with availability of materials, at the expense of spending extra time leveling up.
If you are rerolling on a new realm or on the opposite faction on the same realm as your main, it is worth considering creating a Death Knight purely for the purposes of farming, provided you meet the initial level criteria and do not already have a Death Knight on that realm. Since the character starts at level 55, and enters the standard content at level 58 with a very fast mount they will be able to gather resources such as ore, herbs, leather and even cloth with much greater efficiency than a starting character. Furthermore, having a dedicated farming alt will allow more unusual profession combinations among the player's other characters, such as blacksmithing and jewelcrafting or leatherworking and enchanting. Ideally a farming alt would have two of the three gathering professions, skinning, herbalism and mining and whatever resources are not used by the player's other characters should be sold at auction to provide much needed gold.
A bank alt 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 including a 1-member guild with a guild bank.)
- Have a Character right next to the Auction House to Auction all your stuff if you're far away from one.
- Manage numerous amounts of resources
Some alts exist primarily:
- to be mules (AKA bank alts, very common, usually low level, often level 1)
- to be twinks (fairly common, focused on PvP, does not want to level past their bracket)
- an addon debug alt (AKA debug toon) may be created on a separate realm in order to debug addons that may cause them to lose gold and/or items.
These types of characters are not always played in the normal sense; see their respective pages for more information.
All accounts may have up to 50 characters at a time.
Alts on the same account and faction can be in the same guild and use the same guild bank. If their individual privileges within the guild allows them to, they can trade cash or items through the guild bank; there are no account based restrictions.
Alts on the same account cannot both be present at once.
So alts on the same account cannot trade with each other using the trade window.
Conjured items cannot be sold at the Auction House, mailed nor put in a guild bank tab, but can be given, sold, or traded with other player characters using the trade window. Unfortunately, that means you cannot pass these directly to an alt on the same account.
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 restrictions placed on Alts on the same account, and also lose the benefits. Few players run 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 ((Addons include Friends and Ignore Share, and others - This is actually a default feature of certain games more loosely associated with realms such as Guild Wars))
- 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.
- 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 (A reference to all drug addictions usually defaults to/relates to alcoholism, and World of Warcraft can be a "drug" in a modern brain chemistry sense.)