Ketchup is America’s favorite condiment, being found in 97 percent of kitchens. Studies show tomato ketchup can also be a powerful tool in the fight against cancer and heart disease. You might be surprised to learn that ketchup is not limited to plain tomato varieties.
The word ketchup is derived from the Chinese ke-tsiap, a pickled fish sauce. It made its way to Malaysia where it became kechap and ketjap in Indonesia.
By the nineteenth century, ketchup was also known as tomato soy.
Early tomato versions were much thinner with a consistency more like a soy or Worcestershire sauce.
F. & J. Heinz Company began selling tomato ketchup in 1876. By the end of the nineteenth century, tomato ketchup was the primary type of ketchup in the United States, and, the descriptor of tomato was gradually dropped.
Catsup and catchup are acceptable spellings used interchangably with ketchup, however,ketchup is the way you will find it listed in the majority of cookbooks.
Text from http://homecooking.about.com/