The broad division of Noh plays into the two categories of mugen nō 夢幻能 "Phantasm Noh" and genzai nō 現在能 "Reality Noh" is a useful one, not least because "inventor of mugen nō, and therefore perfecter of the Classical form" is a handy nutshell summary of who Zeami was.

But Zeami and his contemporaries didn't actually use that terminology. In fact, according to Umehara Takeshi's Umehara Takeshi no jugyō: Nō o miru (梅原猛の授業 能を観る "An Umehara Takeshi course in watching nō"), the phrase mugen nō was coined in 1909 1926 by Sanari Kentarō 佐成謙太郎. Umehara claims that in a "Radio lecture on national literature" (国文学ラヂオ講座), Sanari said the following of the Noh play "Yorimasa" 頼政:

私はこのように、劇の主人公がワキの夢に現れてくるものを夢幻能と名づけ、従って『頼政』の如き脚色を複式夢幻能と申せばどうであろうかと思うのでございます

In this way I suppose that one might call those [plays] where the protagonist appears in the waki's dreams mugen nō, and therefore to refer to plays with a structure like "Yorimasa" as fukushiki mugen nō ["two-part phantasm Noh"]

... "Two-part phantasm Noh" being the classic, even stereotypical Noh structure: a first act where the waki encounters a rustic local who obligingly explains the details of some historical tragedy that took place nearby, and a second act where the rustic local returns in his true form: the ghost of said tragedy's principal figure. The name of this structure was also the inspiration for the John Lennon/Yoko Ono album title Double Fantasy. (Sadly, that last sentence may not be entirely true.)