lean vs lean back

lean is a noun but lean back is not a noun.

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

As verbs, lean back is a hyponym of lean; that is, lean back is a word with a more specific, narrower meaning than lean:

Other hyponyms of lean include incline, pitch, slope, weather, heel, list, recline.

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

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

lean (verb)	lean back (verb)
rely on for support	move the upper body backwards and down
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