An answer engine is a system that responds to a query with a direct answer rather than a ranked list of links. The category includes AI assistants like ChatGPT and Perplexity, Google's AI Overviews and AI Mode, and even featured snippets, which all aim to resolve the query in place.
For brands, answer engines are the surfaces that Answer Engine Optimization targets. Because the user often gets a complete answer without clicking, being the source the engine draws its answer from, and being named within it, is what wins visibility.