Titan
The term 'titan' is used in many games to refer to something huge and impressive. It's always a metaphor for the godlike monsters of Greek Mythology that preceded the god-type gods of Zeus et al. The adjective "titanic" is synonymous with "fuckhuge," "Jesus that's big" and "did you say iceberg?"
British Ships
The RMS Titanic was a luxury ocean liner that was the largest vessel of her time. A medium-passage ticket cost about as much as a normal person's wages for six months. It was called "unsinkable," and "too big to fail," much like Enron, the Vietnam War, Communism, and the Star Wars Imperial space stations "Death Star", "Death Star Mk II" and Star-killer base (Death Star Mk.III).
Dungeons & Dragons
See Titan (D&D) for relevant information.
End of War
In End of War, Titans are walking armadas of recycled "Super Heavy Dreadnought Battleships" invented by Hitler in 1946. They also appear in later series as walking HQ's with plasma rays and railcannons to fuck everything until its bleeding and crying
Avalon Hill
A wargame of recruiting monsters so you can recruit bigger monsters, and then catching up to other player's stacks of monsters so you could fight it out on sub-maps. You want to find and kill your opponent's "titan" monsters, which are as powerful as how many points that player has acquired so far. A classic of the monster slug-a-thon genre of boardgames. Someone made a flash version, which is Awesome.
Mechaton Titan
In Mechaton Titan, "Titans" are the largest class of unit that can be made. We're talking LEGO giant robots that are bigger than your head.
Warhammer 40,000
In Warhammer 40,000, Titans are gigantic warmachines. They come in Big, Bigger, Bigger than the Last One, Damn that's One Big Mecha, and XBAWKS HUEG (fucking massive).
They also tend to fuck up your shit. By at least times 5 (or ten).
To elaborate: Titans are the biggest freaking land machines most races have at their disposal in the GRIM DARKNESS OF THE 41ST MILLENNIUM. Other than several types of kilometer to multi-kilometer long landships, but those don't usually leave their planet of origin.
How big are they? Actually, that varies depending on the depiction. For the most part, think of a giant mecha that is 15-80 meters tall. Your typical troopers on the tabletop are half as tall as your finger is long -- a Titan miniature requires both of your hands to lift. Some Titan minis are bigger than your head, which makes the name "miniature" really a misnomer. Sometimes we have particular class that even has a complete city block (in the form of a gargantuan ornate castle) built ON ITS SHOULDERS. It also has rules in Warhammer 40K. To say the "miniature" will be taller than you is an understatement. Just how much bigger though is a mystery because the damn things vary from 40m in the Titan Legions fluff to...something quite obviously taller than 40m when using the infantry pouring out of its legs as a scale guide on the box cover. Go figure. Hehe figure. Talking about miniatures...
How powerful? Take a couple of Yamato-class battleships and add More Dakka and a bunch of other shooty bits. Oh, and some of them have enough nuclear firepower to devastate an entire biosphere of a large island. Or half a continent, depending on which Titan you're being exterminated by. They have almost enough dakka You can NEVER have enough. The largest, the Imperator and Warmonger classses, can devastate entire planets if left unchecked.
How scary are they? The apocalypse is walking towards you; start praying to whatever holy god you believe in and hope it uses the bolter cannon first, cause that flamethrower shit hurts. Though only some of them have models, and you can only get them from Forge World, so, they cost an arm and leg in terms of real money unless you have the time, skill, cardboard, and paper mache to make one yourself.
There is also a planet called Titan (technically a moon, but it's bigger than motherfucking Mercury) in the Terra System. This is where the Grey Knights' highly classified fortress-monastery is, though for most of the Imperium it's just an annoying area of restricted voidspace right in the middle of the Imperium's second largest naval base - the Saturnian moon system. Malcador the Sigillite once moved this planet to the Warp for one year, using nothing but his own mind.