# Semantic index

> The semantic index is the search layer Copilot uses to find relevant content in your Microsoft 365 data by meaning, not only by matching the exact words.

**Category:** Copilot

**The semantic index decides what counts as relevant.** When Copilot [grounds](/glossary#grounding) a prompt, it searches [Microsoft Graph](/glossary#microsoft-graph) through the semantic index rather than with a plain keyword search. Because it works on **meaning**, a question about "the client who was unhappy" can match an email that says "frustrated" or "disappointed". Whatever it turns up is appended to your prompt before the model writes the answer.

You never use the semantic index directly; it runs behind every grounded prompt. It also explains why answers can vary: it ranks what looks most relevant right now, so a renamed or recently active file can change what gets picked. Like the rest of grounding, it only surfaces content you have permission to see.

## At work

A project lead asks Copilot for "complaints about the delivery delay" and gets back a Teams thread where a client wrote that they were still waiting on the shipment, a match on meaning that a keyword search would have missed.

## Often confused with

Keyword search. A keyword search only finds the words you typed; the semantic index also finds content that says the same thing in other words.

## Related terms

- Grounding
- Microsoft Graph
- Context

## See it in a lesson

[Context and grounding](/en/learn/copilot/working-smart/context-and-grounding)

## Further reading

- [Semantic indexing for Microsoft 365 Copilot (Microsoft Learn)](https://learn.microsoft.com/en-us/microsoftsearch/semantic-index-for-copilot): How the semantic index finds relevant content by meaning.
- [Microsoft Copilot architecture and how it works (Microsoft Learn)](https://learn.microsoft.com/en-us/microsoft-365/copilot/microsoft-365-copilot-architecture): Where the semantic index sits in how Copilot handles a prompt.

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Last updated: 14/10/2026  
Source: Rollo Academy glossary