Number one was an old Earth naval term that was used by captains when referring to their first mates. Like many Earth naval traditions, this term carried forward into Starfleet and is used by many captains toward their executive officers.
Among the noted Starfleet captains in the habit of calling their XO "number one" were Christopher Pike and Jean-Luc Picard. (TOS episode: "The Cage"; STA novel: Progenitor; TNG episode: "Encounter at Farpoint"; TNG novel: Q & A)
Known "Number ones"
- Number One
- Commander Michael Burnham
- Commander Saru
- Commander William T. Riker
- Lieutenant Commander Data
- Marissa Flores
- Commander Elizabeth Shelby
- TOS episode: "The Cage"
- DSC episode: "The Vulcan Hello"
- DSC episode: "Context Is for Kings"
- TNG episode & novelization: Encounter at Farpoint
- TNG movie, novelization & comic adaptation: First Contact
- TNG episode: "Peak Performance"
- TNG episode: "Disaster"
- TNG novel: Vendetta
- DSC episode: "The Wolf Inside"