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?"
Greek Mythology
Titans ruled the Earth before the Gods took over Mount Olympus, Titans where enslaved, some are tortured for Eternity while other are forced to do labour, Eg, Atlas holds up the sky and Baneblade crushes noobs.
The USS Titanic was a luxury oceanliner that was the largest vessel to date. 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 Imperial space stations "Death Star" and "Death Star Mk II."
End of War
Invented by Hitler in 1946, Titans are a walking armarda of recycled "Super Heavy Dreadnought Battleships", 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 opponents'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.
Mechaton Titan
Big Lego mecha, I mean, REAL big Lego mecha, very stompy indeed...
Warhammer 40k
They are "sacred war machines" of the Adeptus Mechanicus. They fuck up your shit. They come in big, bigger, bigger than the last one, damn that's one big mecha and WHAT THE HELL IS THAT GIANT MONSTROSITY?! (answer: a baneblade)
They also tend to fuck your shit up. at least 5 times.
To elaborate: Titans are the biggest freaking land machines the Imperium has at it's disposal in the GRIM DARKNESS OF THE 41ST MILLENNIUM.
How big are they? Think 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.
How powerful? Take a couple of Yamato-class battleships and add More Dakka.
How scary? The apocalypse is walking towards you; say prayers to whatever deity you believe in and hope it uses the bolter cannon first, cause that flame thrower hurts.