lassie vs young woman

lassie and young woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
lassie	Yes	No	No	No
young woman	Yes	No	No	No

As nouns, young woman is a hypernym of lassie; that is, young woman is a word with a broader meaning than lassie:

Other hypernyms of lassie include fille, girl, miss, missy, young lady.

lassie (noun)	young woman (noun)
a girl or young woman who is unmarried	a young female