Diameter was the geometrical straight line through a circular object. It could specify the object's dimension.
In June 2151, Captain Jonathan Archer told Andorian Commander Thy'lek Shran that a man in Canton, Ohio had once rolled a ball of string six meters in diameter. (ENT: "The Andorian Incident")
The diameter of "Archer's Comet" was 82.6 kilometers, according to T'Pol's readings. (ENT: "Breaking the Ice")
Following her scans of the debris of the alien ship found in the Arctic Circle, Rooney identified it as spherical and about six-hundred meters in diameter. (ENT: "Regeneration")
In 2265, Lee Kelso measured the unknown object later to be identified as the SS Valiant's ejected disaster recorder to be approximately one meter in diameter. (TOS: "Where No Man Has Gone Before")
In 2266, Spock estimated the diameter of the Fesarius to be one mile. (TOS: "The Corbomite Maneuver")
In 2267, James T. Kirk claimed over a Code 2 transmission that the auto-destruct initiated by a "corbomite device" would destroy all matter within a two hundred thousand kilometer diameter, a clever bluff to fool the Romulans. (TOS: "The Deadly Years")
The nucleogenic cloud being encountered by the USS Voyager in 2371 measured seven au in diameter. (VOY: "The Cloud")