II. Synergist·noun A remedy which has an action similar to that of another remedy, and hence increases the efficiency of that remedy when combined with it.
Wikipedia
Synergist
Broadly, a synergist is an entity that displays synergy with respect to another entity.
More specifically, a synergist may be:
a synergist muscle
a substance that enhances the effect of another substance, such as a drug (see Synergy § Biological sciences)
something relating to the theological position of synergism