Waaagh!
The Orkish term Waaagh refers to three related phenomena. Ork behavior is dominated by the Waaagh!, a psychic field they generate. This field allows Orks to instinctively recognize who is 'bigga' and therefore, in charge. This field also has some effect on reality; it plays a key role in making many Orkish technologies work at all, and lends truth to certain Orkish beliefs, such as the idea that red vehicles move faster than those of other colors. Orks seem unaware the Waaagh creates these effects, simply believing they are how the world works.
Waaagh is also the name of the rare times when Orks band together to conquer and destroy other races rather than fighting themselves. Finally, it is the name of the massive horde of Orks gathered under a single Warboss - an individual Ork who is invariably the largest, most dominating and physically intimidating. He is able to unite the different warbands of Orks into one massive, almost unstoppable horde - almost always for the purpose and duration of a Waaagh in the second sense.
The occasion of a Waaagh is a cross between a mass migration, holy war, looting party and pub brawl, with a bit of genocide thrown in for good measure. Millions of battle-hungry Orks invade their target and stomp everything in their way flat. Just who is invaded is a matter of supreme indifference to the Waaagh. Battle is battle.
Waaaghs naturally gain in size and impetus as long as they're successful and have something to fight. As the Waaagh rages on, more Orks will hear of the fighting and travel to where the action is. Thus, the longer a Waaagh goes on the more numerous it becomes.
A Waaagh is typically dispersed at its defeat, the death of its Warboss or simply when it runs out of enemies. The combined warbands and tribes will then break up through infighting and inter-clan rivalry. Exceptions do occur- a particularly dominating Warbosses occasionally manages to hold his boyz together, forging for himself a small empire. The best example of this is the Ork empire of Charadon, which has survived for thousands of years under the rule of successive Warlords.
Waaagh energy can also be manipulated by the Weirdboyz in an array of deadly physic powers.
The current most notable instigator of Waaaghs is Warlord Ghazghkull Thraka, responsible for the Second and Third War for Armageddon.