union, sum, join -- (a set containing all and only the members of two or more given sets; "let C be the union of the sets A and B")
Overview of verb join
The verb join has 5 senses
join, fall in, get together -- (become part of; become a member of a group or organization; "He joined the Communist Party as a young man")
join, bring together -- (cause to become joined or linked; "join these two parts so that they fit together")
join -- (come into the company of; "She joined him for a drink")
join, conjoin -- (make contact or come together; "The two roads join here")
connect, link, link up, join, unite -- (be or become joined or united or linked; "The two streets connect to become a highway"; "Our paths joined"; "The travelers linked up again at the airport")