A Joining was a term used by the Taelons which resulted in two species blending together on a biological level.
During the Companion's time on Earth, there was much debate on what to do with the Human race. Synod member Zo'or was of the opinion that the species needed to be subjugated and bioengineered as a weapon to use against their mortal enemies the Jaridians. However, Da'an felt that a Joining of the two races was a more worthy cause which would aid their race in reclaiming something lost in their evolution though there were doubts on whether mankind was evolved enough for such a process to occur. (Episode: The First of its Kind) Da'an believed that Humans needed to be elevated to the point that a Joining could occur. (Episode: Moonscape)