midline vs midplane

midline and midplane both are nouns.

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

As nouns, midline and midplane are synonyms defined as:

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

midline (noun)	midplane (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)