A composite thing is understood as constituted by parts; a noncomposite thing is not so constituted. Within the category of substance, this opens two distinct discussions: body (jism) and indivisible substance (jawhar fard).
Composite substance: parts combined in a unity called a body.
Noncomposite substance: in the theological model, an elementary part not divisible further.
This is not an assertion that a modern physical atom is identical to the classical jawhar fard. The latter belongs to a specific metaphysical and logical account of composition.