n (countable) The act of uniting or joining two or more things into one.
n (uncountable) The state of being united or joined.
n (countable) That which is united, or made one; something formed by a combination or coalition of parts or members; a confederation; a consolidated body; a league.
n (countable) A joint or other connection uniting parts of machinery, such as pipes.
n (countable|set theory) The set containing all of the elements of two or more sets.