concatenation vs joining

concatenation and joining both are nouns.

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

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

concatenation: the act of linking together as in a series or chain
joining: the act of bringing two things into contact (especially for communication)

Other hypernyms of concatenation include connection, connexion.

concatenation (noun)	joining (noun)
the act of linking together as in a series or chain	the act of bringing two things into contact (especially for communication)
a series of things depending on each other as if linked together
the linking together of a consecutive series of symbols or events or ideas etc
the state of being linked together as in a chain; union in a linked series