Ketchup is the thick, cold, sweet-sour red sauce that comes out of a squeeze bottle onto chips, burgers, sausages, and eggs. In the United States it is almost always tomato ketchup, and people put a lot of it on food; in Britain the same red stuff is ketchup too, sitting next to brown sauce, which is a different bottle. That red picture is what learners meet. Though actually, ketchup did not start as a tomato sauce at all. English speakers first used the word in the late 1600s for a salty sauce they met in Southeast Asia — closer to a thin fish or soy sauce than to the red stuff we know. Tomato went into the bottle much later, and for a long time British cooks also made ketchup from mushrooms or walnuts. You can still buy mushroom ketchup in some British shops; it is dark and thin, not red. So the tidy idea “ketchup = tomatoes” is the modern habit, not the whole history. Some American bottles still say catsup, which is the same sauce with an older spelling, not a different food. It is an everyday table word, not formal. It is not chilli sauce, which is hot, and it is not the pasta sauce in a jar.