Coca-Cola was a brand of cola offered in the 20th century on Earth.
In 1986, a eatery on Columbus Avenue advertised that they were selling this product. Later, when James T. Kirk, Leonard McCoy, and Gillian Taylor were on a rescue mission to save Pavel Chekov from that century's doctors, there was a can of this cola at a nurses station.
The can was labeled as having the official formula. This advertisement marked a return to a formula that had proved popular with customers after a failed experiment with New Coke in 1985.
Another Coca-Cola product, stocked by Dr. Nichols, was a diet version called Diet Coke. (Star Trek IV: The Voyage Home)