Definition-first content is the practice of opening every chunk on a page with a one-sentence, sub-forty-word direct answer to the question the heading implies, before any context or example. It is the single highest-leverage edit you can make for AI citation odds. Pages that follow this pattern across all sections out-cite their direct competitors by a wide margin even when nothing else about the content changes.
Why this pattern wins retrieval
Retrievers chunk pages with overlap, so the first 150 tokens of a chunk dominate that chunk's embedding. If your first sentence is the answer, the chunk embeds tightly to the buyer query. If your first sentence is windup ('In recent years, the conversation around X has shifted...'), the chunk embeds loosely and a competitor whose answer is at the top of their chunk wins. Definition-first is essentially answering the retriever's question before the LLM ever sees the chunk.
The 40-word rule
Aim to keep the opening definition under forty words. That length fits comfortably inside a featured snippet, an AI Overview citation, and a Perplexity quote. Above forty, the answer engine is more likely to truncate it, which lowers the odds that your phrasing is the one cited. Below twenty, you usually have not said enough for the engine to be confident in the chunk. Twenty-five to thirty-five words is the practical sweet spot.
Before and after
Before: a section under the heading 'What is mention rate' opens with 'Mention rate is one of those metrics that has come up a lot recently as more buyers turn to AI assistants for advice.' That sentence has zero answer content. After: 'Mention rate is the percentage of AI answers that name your brand for a given buyer question, sampled across many runs to get past the noise of stochastic models.' That sentence is the answer. The retriever can stop reading there, and so can the human.
Where to apply this first
Apply it to every H2 and H3 on the pages you most want cited: comparison pages, category-defining glossary entries, pricing pages, and the FAQ section of high-intent landing pages. Re-read each section's first sentence. If it is windup, replace it with the literal answer. The whole audit usually takes under a day for a 30-page site, and the lift in citation share shows up within two weeks.
What this does not replace
Definition-first does not replace the rest of the section. The follow-on paragraphs still need substance, examples, entities, and the supporting argument — all the things that make the page useful to a human reading it after the AI sends them there. Definition-first is about the first sentence. The rest of the section is the reason a clicker stays.