# Glossary of Key Terms in Answer Engine Optimization (AEO)

> Canonical source: [Glossary of Key Terms in Answer Engine Optimization (AEO)](https://www.answer.cloud/pages/lexicon-for-answer-engine-optimization-aeo-hosted/)

Definitions for AEO, Agent2Agent, retrieval, crawler preferences, citations, and AI referrals.

## AEO — Answer Engine Optimization

The practice of making useful information discoverable, understandable, and verifiable in AI-assisted answers. Results depend on the system and question.

## SEO — Search Engine Optimization

Work that helps search engines discover and understand pages and improves their usefulness for relevant searches. It remains relevant to AI products that use search.

## A2A — Agent2Agent

An open protocol for communication and collaboration between agents. It is not “Answer-to-Answer” and is not a competing website content-index format. See the [official A2A project](https://a2a-protocol.org/).

## llms.txt

A proposed Markdown guide to a website’s relevant content and text resources. It does not define crawl permissions. See the [primary proposal](https://llmstxt.org/).

## robots.txt

A file expressing crawl preferences to cooperating robots. It is not authentication and does not itself prevent an already known URL from being indexed. Use appropriate indexing controls and real access protection for their separate purposes.

## XML sitemap

A list of URLs and optional metadata that helps supported consumers discover pages. It does not guarantee indexing or an AI citation.

## Retrieval-augmented generation (RAG)

A design that retrieves relevant source material and supplies it as context for generation. Implementations and document sizes vary.

## Query fan-out

Using related searches to explore parts of a question. Some search-enabled AI systems use this technique; illustrative question suggestions are not a log of a provider’s hidden searches.

## Citation, referral, and crawler request

A citation is a source reference in an observed answer. A referral is a visit arriving from another service. A crawler request is access to a URL. These signals measure different events.

See the [metrics guide](https://www.answer.cloud/pages/llm-visibility-metrics-the-kpi-playbook-for-seo-and-marketing-leaders-hosted/) for practical measurement definitions.