noun
(uncountable, chiefly US, Canada, UK) Ellipsis of tomato ketchup (“a tomato-vinegar-based sauce, sometimes containing spices, onion or garlic, and (especially in the US) sweeteners”).
(countable, now rare) Such a sauce more generally (not necessarily based on tomatoes, but with mushrooms, fish, etc.). This is the older meaning.
verb
(transitive) To cover with ketchup.