In a parallel universe, assimilation was the process by which the Borg converted their victims into their kind. In the 24th century, Captain Jean-Luc Picard was abducted and brought aboard a Borg cube where he was assimilated, becoming known as "Locutus of Borg". His knowledge of Starfleet tactics and weaknesses allowed the Borg to easily destroy a Starfleet task force at Wolf 359. However, Picard was ultimately rescued by Lieutenant Commander Data and Lieutenant Worf, who returned him to the Enterprise where the assimilation process was undone.
The Borg attempted to assimilate the Cybermen. However, they found that to do so was impossible and so deemed it necessary to eradicate the Cybermen. In light of their encounter with the Eleventh Doctor, the Borg sought to investigate and master time travel in order to assimilate the Time Lord.
Assimilation was similar to the cyber-conversion process performed by the Cybermen, though the Doctor noted that Borg assimilation was much faster. (COMIC: Assimilation²) Indeed, in the Doctor's universe, the term "assimilation" was occasionally applied to cyber-conversion. Jack Harkness warned Ianto Jones that even a single Cyberman could lead to the assimilation of entire populations. (TV: Cyberwoman) In 102, an empty Cyberman body told Amy Pond that she would be assimilated as it attempted to convert her, only to be killed by Rory Williams. (TV: The Pandorica Opens)