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Core concept · AI Search Infrastructure

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Definition

Topic coherence is the degree to which all content within a chunk or section addresses the same underlying topic. High topic coherence produces tight embeddings that retrieve consistently for their target sub-query. Low topic coherence produces diffuse embeddings that retrieve weakly across multiple sub-queries. Topic coherence is the content-side property that determines embedding quality. Research confirms that explicitly incorporating topic structure into embedding construction reduces retrieval of off-topic chunks by statistically significant margins — and that the penalty for low-coherence passages increases as embedding models improve. The models are getting better at detecting it, not worse.

Semantic center of gravity

Embedding drift

Semantic density

Vocabulary effect

Cluster

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