An original or primary metaphor (asliyya) occurs when the borrowed expression is an underived noun. The new meaning is carried by that noun directly.
Using “darkness” for misguidance and “light” for guidance illustrates this type: the nominal terms themselves carry the metaphorical reference.
This differs from the tasrihiyya/makniyya distinction. The latter asks which comparison term is expressed; asliyya/tab'iyya asks what kind of expression bears the transfer.