Ketchup, also known as catsup, is a type of sauce containing spices and a large amount of acid (vinegar or lactic acid), typically tomato-based. Some recipes, particularly those from Australia, may use the ambiguous term tomato sauce to mean tomato ketchup.
Ketchup is commonly used on deep fat fried foods, meat, and eggs. Ketchup is normally applied at the table as a condiment, but it is also commonly used as an ingredient for meatloaf and barbecue sauce.
Interestingly, the original ketchup did not include tomatoes. It was probably a Chinese sauce called "ke-tsiap", based on fermented brined fish or shellfish. The sauce was brought back to Europe by sailors, and appropriated into local recipes. Later, tomatoes were added.