In the television series Star Trek: The Next Generation, Captain Jean-Luc Picard (Patrick Stewart) often referred to Commander William Thomas Riker (Jonathan Frakes), the second-in-command of the ship, as "number one" due to Riker being the first officer. This may be the reason Majel Barrett's character in the original series' pilot was called "number one" as well.
Calling his first officer this was ingrained into Picard's routine enough that when he was assimilated into the Borg collective as Locutus, he still referred to Riker as "number one" during a communication between the Enterprise and the Borg cube.