approximation vs joining

approximation and joining both are nouns.

Word	Noun	Adjective	Verb	Adverb
approximation	Yes	No	No	No
joining	Yes	No	No	No

As nouns, joining is a hypernym of approximation; that is, joining is a word with a broader meaning than approximation:

approximation: the act of bringing near or bringing together especially the cut edges of tissue
joining: the act of bringing two things into contact (especially for communication)

Other hypernyms of approximation include connection, connexion.

approximation (noun)	joining (noun)
the act of bringing near or bringing together especially the cut edges of tissue	the act of bringing two things into contact (especially for communication)
the quality of coming near to identity (especially close in quantity)
an approximate calculation of quantity or degree or worth
an imprecise or incomplete account