In later centuries, this is the term for the injectors in each nacelle that are fed the warp plasma coming off the warp reactor core in order to fire it through the warp coils. On the U.S.S. Enterprise-D, Daniel Kwan killed himself under an memory echo's influence by jumping into the injectors' plasma stream.
On the NX-01 Enterprise of Archer's alternate future reality erased by curing his multiphasic parasites, plasma injectors are located in an assembly just behind a vertical bulkhead hatch on the port wall of Main Engineering (although these appear to be more precisely the matter injectors). Damaged injectors will not allow the warp reactor to initialize. Spare injectors are kept in a supply locker on the upper level of Main Engineering.