A stellium is defined as a cluster of three (or, if the moon or sun are involved, four) or more planets in a single sign or house.
Only physical bodies are included in stelliums, not points like the ascendant or any other angle, the Moon's nodes, the vertex, any of the parts, like the part of fortune. Only planets are included in the definition of a stellium.
Groups of planets, points, asteroids, stars in a single sign or house are clearly influential; they just aren't classified as a stellium.
Hope this helps clarify.