lean back vs lean

lean back is not a noun while lean is a noun.

Word	Noun	Adjective	Verb	Adverb
lean back	No	No	Yes	No
lean	Yes	Yes	Yes	No

As verbs, lean is a hypernym of lean back; that is, lean is a word with a broader meaning than lean back:

Other hypernyms of lean back include angle, slant, tilt, tip.

lean back (noun)	lean (noun)
	the property possessed by a line or surface that departs from the vertical

lean back (adjective)	lean (adjective)
	lacking in mineral content or combustible material
	lacking excess flesh
	not profitable or prosperous
	containing little excess

lean back (verb)	lean (verb)
move the upper body backwards and down	rely on for support
	cause to lean or incline
	to incline or bend from a vertical position
	cause to lean to the side
	have a tendency or disposition to do or be something; be inclined