Assimilation was the process in which a person learned or assumed the values, behaviors, beliefs, ideas, or other cultural aspects and information of a society.
In 2365, K'Ehleyr beamed aboard the 23rd century ship IKS T'Ong and began the assimilation of the Klingon crew to their era. (TNG: "The Emissary")
In 2372, Michael Eddington stated that the United Federation of Planets was in some ways worse than the Borg because the Borg told others about their plans for assimilation, while the Federation was more insidious and assimilated people without them even knowing it. (DS9: "For the Cause")