A web was a flap of skin in between one's digits.
Moto, a Starfleet Security officer who served in USS Enterprise's brig in 2259, had webbed hands. (Star Trek Into Darkness)
In 2269, as a result of the Aquan surgo-op, James T. Kirk and Spock developed webbed hands. (TAS: "The Ambergris Element")
Megazoids had webbed feet. In 2293, Crewman Dax showed his feet to prove that he could not have been the wearer of a pair of magnetic boots used by one of Chancellor Gorkon's assassins. (Star Trek VI: The Undiscovered Country)
Zaldans had webbed fingers. In 2364, during his entrance examinations, Wesley Crusher recognized Rondon as one, averting a breach of cultural etiquette and successfully passing a deliberate cultural awareness test. (TNG: "Coming of Age")
Malcorians had webbed hands and termini. (TNG: "First Contact")