lemon vs lemon tree

lemon and lemon tree both are nouns.

Word	Noun	Adjective	Verb	Adverb
lemon	Yes	No	No	No
lemon tree	Yes	No	No	No

As nouns, lemon and lemon tree are synonyms defined as:

lemon and lemon tree: a small evergreen tree that originated in Asia but is widely cultivated for its fruit

Other synonyms of lemon include Citrus limon.

lemon (noun)	lemon tree (noun)
an artifact (especially an automobile) that is defective or unsatisfactory	a small evergreen tree that originated in Asia but is widely cultivated for its fruit
a strong yellow color
a distinctive tart flavor characteristic of lemons
yellow oval fruit with juicy acidic flesh
a small evergreen tree that originated in Asia but is widely cultivated for its fruit