"having one form only," by 1839, from mono- "one, single" + -morphic, from Greek morphē "form, shape" (see morphic).
Entries linking to monomorphous
mono-
word-forming element of Greek origin meaning "one, single, alone; containing one (atom, etc.)," from Greek monos "single, alone," from PIE root *men- (4) "small, isolated."
morphic adj.
in biology, "of or pertaining to form," 1826, from Greek morphē "form, shape," a word of uncertain etymology, + -ic.