sick leave vs leave of absence

sick leave and leave of absence both are nouns.

Word	Noun	Adjective	Verb	Adverb
sick leave	Yes	No	No	No
leave of absence	Yes	No	No	No

As nouns, leave of absence is a hypernym of sick leave; that is, leave of absence is a word with a broader meaning than sick leave:

sick leave: a leave of absence from work because of illness
leave of absence: the period of time during which you are absent from work or duty

Other hypernyms of sick leave include leave.

sick leave (noun)	leave of absence (noun)
a leave of absence from work because of illness	the period of time during which you are absent from work or duty