An Incarnation is the singularly powerful manifestation of a spirit-entity that represents a singular concept. While an Avatar is a similar manifestation of a divine being, an incarnation does not form from another sentient entity, such as a god. It is the force of a concept itself.
Incarnations typically do not have names aside from their core concept. Also see Spirit.
Incarnations are also known as Kami in settings based on Japanese culture; however Kami also refers to full deities and minor spirits (there doesn't seem to be a distinction in Japanese folklore). Kami that are singularly powerful (and generally unique), but without a distinguishing name should be Incarnations; those powerful kami with distinguishing names should be considered a Deity. Those that are less powerful, or not unique, should be considered a type of Spirit.