Channel 1 was a communications channel used by the USS Enterprise.
In 2267, as Captain James T. Kirk and the Gorn captain fought on a planet, Spock ordered Uhura to leave this channel on, just in case. (TOS: "Arena")
Later that year, after returning to the Enterprise following their encounter with the Horta on Janus VI, Spock informed Captain Kirk that Chief Engineer Vanderberg was standing by on channel 1. (TOS: "The Devil in the Dark")