midplane vs midline

midplane and midline both are nouns.

Word	Noun	Adjective	Verb	Adverb
midplane	Yes	No	No	No
midline	Yes	No	No	No

As nouns, midplane and midline are synonyms defined as:

midplane and midline: the median plane of the body (or some part of the body)

midplane (noun)	midline (noun)
the median plane of the body (or some part of the body)	the median plane of the body (or some part of the body)