Jon was an alias used by Captain Jonathan Archer when he was asked by the Akaali apothecary Riaan about his name. (ENT: "Civilization")
The nickname was also used by his friends and colleagues (ENT: "Broken Bow", "Fortunate Son", "First Flight", "Carpenter Street", "Home", "Daedalus"), and occasionally, adversaries. (ENT: "Broken Bow", "Cold Front")
Captain Jon Daily also shared the name "Jon", which James T. Kirk used to address him in 2266. (TOS: "The Conscience of the King")