What is a Wombo Combo?

I'd like to know what exactly the term 'Wombo Combo' means.

Does it mean something such as using all 5 champion's ultimates at the same time?

Is it a term only used in League of Legends?

Edit

So according to this site the origin of the 'Wombo Combo' term comes from Super Smash Bros Melee (amazing game too) to define an "crazy combo".

For LoL does it means instakill everyone?

What are the famous 'Wombo Combo' of LoL?

The origin of the "Wombo Combo" comes from Super Smash Bros Melee, as you stated in the question, and it is defined as two players in the game chain together certain moves to highly damage and/or kill the enemy. The moves compliment each other and thus create a combo.
In League of Legends it means the same, to combine moves (not necessarily a champions ultimate only) to create a very strong "combo" attack with your teammate(s).

What are the famous Wombo Combo of LoL?

They are ever changing and new ones being made, but some examples are:

• Blitzcrank's Power Fist to knock someone up, followed by Yasuo's Last Breath to deal the damage.
• Orianna's Command: Shockwave to bring everyone together then Jarvan IV's Cataclysm to keep them there.

Wombo Combo is a term used, primarily, to mean the complete and utter destruction of another person or team, especially if it is done quickly. It is originally from a video of a Super Smash Bros Melee match (below), but has since grown in popularity and is used for many different games and situations to the same effect. There are a large number of Youtube videos that use the same audio track and overlay other situations, gaming or not, as a parody of the original video.

Usually when used in games these days, it is done so in celebration or as a taunt or bragging to the other team. Most of the time, it is just meant as trash talk and is not referring to any action in particular as a Wombo Combo.

Potentially related, it is also a Pizza Combination at Round Table Pizza with "Primo pepperoni, Italian sausage, linguica, bacon, mushrooms, tomatoes, artichoke hearts, green onions on zesty red sauce."