Ketchup appears to be the original term which is derived from the Chinese condiment ke-tsiap, a pickled fish sauce. A version in Malaysia then became kechap and ketjap in Indonesia.
The word ketchup was first mentioned in Charles Lockyer’s book of 1711, An Account of the Trade in India: “Soy comes in Tubbs from Jappan, and the best Ketchup from Tonquin; yet good of both sorts are made and sold very cheap in China”.
The modern version of ketchup saw many changes before tomatoes finally became a main ingredient in the late 1700′s. In the 1800s, ketchup was also known as tomato soy.
F. & J. Heinz Company began selling tomato ketchup in 1876. Heinz brought the preferred term to the forefront with what is one of the most successful versions of the condiment. Heinz once offered ketchup in various colors, including purple.
The confusion about names started even before Charles Lockyer wrote about it, since there is an entry dated 1690 in the Dictionary of the Canting Crew which gives it as catchup. Catchup was used much more in North America than in Britain, but catchup is not used much these days.
The term catsup first appeared in a quote by Jonathan Swift in 1730: “And, for our home-bred British cheer, Botargo (fish relish), catsup, and caveer (cavier)”.
Hunt's goes three ways by having Hunts Catsup (east of the Mississippi), Hunts Ketchup (west of the Mississippi), and Hunts Tomato Cornchops (in Iowa only).
The term catsup has since been used by major manufacturers, but most eventually changed to ketchup. Catsup is an acceptable spelling used interchangeably with ketchup. However, ketchup is the way you will find it listed in the majority of cookbooks.