Mitogen-activated protein kinase (MAPK) is a generic name for a family of different but related enzymes involved in this specific signalling cascade. RAF is a MAPKKK, MEK is a MAPKK and ERK is a MAPK. This is evident in the Nature article from which your image is apparently adapted:
> Activated RAS triggers the activation of the MAPKKK RAF. Activated RAF then phosphorylates the MAPKK MEK (MAPK and ERK kinase) on serine residues leading to its activation. Subsequently, activated MEK catalyses the dual phosphorylation of the MAPK ERK.