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?"
The S.S. Titanic was a luxury ocean liner 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."
Dungeons & Dragons
- Original D&D had them as not-ugly, not-stupid giants in the Greyhawk suppliment.
- AD&D had them in the Monster Manual, 17-22 hit dice and 7d6 points of damage on a normal attack, could cast any magic-user or cleric spells as if level 15, had eight psionic powers but were immune to psionic attack, and invisible at-will. They looked like beautiful 18-foot tall Greek giants. Levitate or ethereal twice a day.
- AD&D2 had them in the Monstrous Compendium #8 (nobody I knew kept buying the MCs after #3, no idea what 2E titans are like.)
- D&D 3 had them in the Monster Manual as 25-foot perfect Greek-god humanoids with a dozen at-will arcane & divine spells as if level 20, and had a challenge rating of 21 (the Tarrasque is a 20). Levitate at-will, ethereal or gate twice a day.
- D&D 4 uses titans as giants that are more elemental-y and 3 levels higher than normal giants. And some how "necrotic" became an elemental thing. They're even uglier than giants, and these titans can't levitate.
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 the fuck huge warmachines of the 40k universe. Basically a giant fucking ripoff of those futeristic anime and manga comics about giant robot mechs, except they got 5 times the grimdarkness. They come in big, bigger, bigger than the last one, damn that's one big mecha,and WHAT THE HELL IS THAT GIANT MONSTROSITY?!
They also tend to fuck up your shit. By at least times 5.
To elaborate: Titans are the biggest freaking land machines most races have at their disposal in the GRIM DARKNESS OF THE 41ST MILLENNIUM.
How big are they? 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.
How powerful? Take a couple of Yamato-class battleships and add More Dakka. Oh, and some of them have enough nuclear firepower to exterminatus an entire planet.
How scary? The apocalypse is walking towards you; start praying to whatever deity you believe in and hope it uses the bolter cannon first, cause that flamethrower shit hurts.