A symbol was an image or object that represented another.
In 2268, Lieutenant Commander Spock was expected to report to sickbay but there wasn't time as he had to decipher the symbols of the Preservers' obelisk which were a highly advanced form of cipher writing. Spock believed the symbols were the key to providing rescue to Captain James T. Kirk and the Amerind inhabitants. (TOS: "The Paradise Syndrome")