In khayālī, sensible components are combined into an externally unreal composite. In wahmī, a sensible-looking feature is attributed to a supposed or nonexistent thing, although no externally verified instance of that thing has been perceived.
Khayālī example: an elaborate poetic scene built from known visible objects.
Wahmī example: teeth ascribed to an imagined ghoul.
Both involve the mind’s constructive activity, but combination of sensed parts is particularly salient in the former, and attributing sensible form to a supposed entity in the latter.