Tank

A French Renault FT-17 Tank, the first tank to have the rough layout that would be the norm for tanks (Crew in the front, top mounted 360 degree turret for main gun, engine in back)

A tank is a tracked, armored combat vehicle. The term is often limited to vehicles intended for direct combat, as opposed to e.g. self-propelled artillery (which stay to the rear) or armored personnel carriers and infantry fighting vehicles (which are on the front line but are primarily tasked with carrying soldiers as opposed to fighting directly). Their invention revolutionized warfare in the 20th century, and any wargame set in or after that time period (or in alternate universes with similar or more advanced technology levels) will have plenty of attention devoted to them -- or to whatever made them obsolete, as in e.g. BattleTech.

The idea of an armored fighting vehicle dates back at least to Leonardo da Vinci and H. G. Wells, but the modern tank was invented shortly before World War I, and was then spurred to production by the war itself. When the war on the Western Front got bogged down in trenches, the British Royal Navy had the idea to use tracked, armored vehicles with guns to break the stalemate. The name "tank" became attached to the vehicle as a codename to disguise the purpose of the large metal bodies being built. After the first tanks rolled onto the battlefield, other countries called them "battle wagons", "armors", "assault vehicles", and other more descriptive names, but the Anglosphere was stuck with calling them "tanks". (Interestingly, the original British Tank, which looked like a Leman Russ but without the turret, was called a "Little Willy")

Tanks were built with pretty much any set of features you could imagine, but over time, the militaries of the world settled on several key features:

1. A large-caliber primary cannon for destroying enemy vehicles and defenses. This is why the Navy was the first branch of the British military to design and produce tanks during WWI.
2. A turret to house the tank gun, to allow the tank to shoot at targets without having to pivot the entire vehicle. The French had this one figured out by 1918, and some tanks developed during the interwar period actually had more than one (though this proved impractical).
3. A sloped, heavily-armored front face to absorb attacks, including those from enemy tanks. This in turn spurred the development of tank destroyers with even bigger guns at the cost of mobility, armour, or overall tactical flexibility (i.e. an inability to rotate their guns more than a few degrees to either side). It also led to the development of anti-tank weapons like missiles and mines specialized to attack more vulnerable rear, underside, and top armor.
4. Tracks with a profile as low as possible while meeting all-terrain mobility needs. A stopped tank is a dead tank, and running the tracks over the top of the body is begging for a mobility kill, no matter how cool the British Mark I looked.

Since Games Workshop seems to think that tank development in the Warhammer 40,000 universe stopped somewhere between the world wars, most of the iconic fighting vehicles of the Imperium lack some of these features. We can quibble that some of those vehicles are not really tanks but armored personnel carriers (e.g. the Land Raider) or other specialized classes of armored fighting vehicles, but GW's design team has a serious problem with looping their tracks all the way up and over the chassis for that 1918 flavor.

Console RPG Character Role

In many role-playing video games, particularly the online ones, the term "tank" has also arisen to describe a character whose primary purpose is being punched in the face.

...That requires some explanation.

See, many enemies in RPGs have way too much health, and fights with them are unlikely to be decided in one round unless they're uncharacteristically vulnerable to save-or-die rays (which almost never happens). Furthermore, many of the classes that are best at killing things are the worst at not getting killed, because that's how balance works. Thus, demand is created for a character whose job is to redirect enemies' aggression away from the squishy members of the party and towards him- or herself, usually using their mastery of mind-control, irritating sound effects, imposition of dangerous effects for attacking anyone else, or simply cutting insults and rude gestures which draw attention to themselves. They also tend to have abilities that help them in resisting, mitigating, avoiding, or regenerating from some of the damage they suffer (and on occasion act as a secondary damage dealer).

4th Edition Dungeons and Dragons refers to this role as the "defender," while Dawn of War 2 vets will recognize it as the "Tarkus", and later the "Diomedes." Most tanks can deal some damage, or even good damage, but it isn't their primary role.