Actually, Erythema refers simply to redness of the skin in its medical context. One would say "Erythema was noted surrounding the lesion."

Erythematous, the adjective form of the same root word, is also appropriate to describe redness, as in "The upper extremity was erythematous distal to the elbow."

Therefore, Erythema Multiforme refers descriptively to the red (erythema) heterogeneous (multiforme) rash that is characteristic of this particular disease.