Not an aspect at all: a concentration. Three or more bodies land in the same sign, which packs a large share of a chart's aspects into a small run of degrees.
A stellium is the simplest configuration to detect and the easiest to overstate, because it isn't really one geometric claim — it's a count. Any three bodies within one 30° span will be within 30° of each other, which puts most pairs inside a conjunction orb almost by construction. The 'concentration of energy' the tradition describes is a real effect of three or four bodies aspecting each other tightly, but the stellium label itself is a bookkeeping device for that, not a fifth kind of aspect.
Because membership is by sign rather than by a fixed angular window, a stellium can be as tight as a few degrees or as loose as the low-20s (three bodies at 2°, 15° and 28° of the same sign share a sign but the outer two are 26° apart — nearly a sesquiquadrate's worth of separation, and not remotely conjunct). This site does not draw a distinction between those cases; the aspect grid alongside it will.
A stellium says relatively little about which two bodies are actually tight and which are merely sign-mates. Read the aspect table for the pairwise orbs before treating three planets in one sign as three planets doing the same thing.
Traditionally attributed
A concentration of emphasis in whatever the sign and house govern — traditionally read as a life organised unusually strongly around one theme.
Listed because it is what the tradition says, not because this site can check it. Everything above this line is geometry; everything in it is convention.