The term gank is used to described any PvP encounter where one side of the fight has no real chance to fight back and defend themselves. The term is most often used when there's a high level disparity between both fighters, such as when a level 70 kills a level 20 (usually in a single blow), but it can also be applied when a group of player team up on another (Such groups are sometime formed with the purpose of ganking and are often called 'gank squad'). Ganks rarely occur on PvE servers where the flagging rules make this a rare circumstance.
Another use of the term is to describe being attacked by another player while fighting mobs and being on low health/mana because of it, giving you little chance to fight back even if you are of the same level as your attacker.
Stranglethorn Vale is infamous for being a ganking ground on PvP servers, the zone is huge and filled with many quests, as well as have many terrain obstacle, making it easy to surprise others. Many PvP players consider leveling through Stranglethorn Vale to be a rite of passage, since it is quite often the most brutal zone on the server as far as gankings go.