It's kind of clear from context, but still this is the first I hear the term (as someone who played those games) and DDG also doesn't come up with anything useful (top result, for example, is about some item in Dragon Ball). So I was also wondering if I got it right, especially when there's a hundred non-jargon and equally concise ways to say that, e.g. 'large group'. To me this seems like a valid question.
It's a common term in modern RTS, and it's got more meaning attached to it than simply "a large group". For example, in StarCraft 2, a full army of zerglings wouldn't be considered a "death ball" because it'd be so easily defeated with a few Colossus and high templars (units that deal a lot of splash damage, which zerglings are particularly susceptible to). A death ball is an army so powerful it's virtually unstoppable once it's formed. Players can try to achieve a death ball to gain a victory, but it's often expensive and leaves you vulnerable while you're building one. Also death balls are usually slow, and the opponent can use faster units to attack vulnerable parts of their base while the player building a death ball is out of position.
My use of the term originates from the Starcraft 2 culture. Search on "Protoss Deathball", which is a well known, highly effective "tactic"... if a bit obvious.
I'm sure the other games have their own terminology for the evolution of this "tactic".
AoE2 also uses this term extensively especially within the context of ranger infantry. A clump of forty arbs will chew through the armor of even intended counters (like knights).
One of the more interesting applications of this term I've seen is in TW WH2 (Total War: Warhammer 2) where a faction like Vampire Counts gets a lot of utility out of death-balling even trash tier units like skeleton warriors - so that opponent infantry and cavalry gets mired in the mass of unit collisions to make it easier to get a clean spell targeting off. I think the term is pretty well established at this point as evidenced by the derivative usages that are now popping up.
