From Looney Pyramid Games Wiki
These three pyramids form a group.

A piece is said to be part of a group if it is touching any of the other pieces in that group. For instance, let's say a red piece is touching both a blue piece and a green piece, but the blue and green aren't touching. Those pieces make up a three-piece group.

A piece that is touching no other pieces makes up a one-piece group and is sometimes affectionately known as a hermit.