Definition
AEO (Answer Engine Optimization) is optimizing content so that answer engines can extract a correct, self-contained answer from it and attribute that answer to you. It overlaps heavily with GEO; where the two are distinguished, AEO emphasises the extractable answer block and GEO emphasises whether the brand gets named and recommended.
Why it matters
An assistant will only lift a passage it can use without the surrounding page. Content written as a narrative that only makes sense in order gives it nothing to lift, however good the writing is.
How to measure it
For each question you want to own, check whether one paragraph on your page answers it in 40 to 80 words without referring to anything above it. Then check whether that paragraph is what assistants actually quote back.