> ## Documentation Index
> Fetch the complete documentation index at: https://docs.metriport.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Expression

> Base StructureDefinition for Expression Type: A expression that is evaluated in a specified context and returns a value. The context of use of the expression must specify the context in which the expression is evaluated, and how the result of the expression is used.

## Properties

| Name        | Required | Type                                                   | Description                                                |
| ----------- | -------- | ------------------------------------------------------ | ---------------------------------------------------------- |
| id          | No       | string                                                 | Unique id for inter-element referencing                    |
| extension   | No       | [Extension](/medical-api/fhir/data-types/extension)\[] | Additional content defined by implementations              |
| description | No       | string                                                 | Natural language description of the condition              |
| name        | No       | id                                                     | Short name assigned to expression for reuse                |
| language    | Yes      | code                                                   | text/cql / text/fhirpath / application/x-fhir-query / etc. |
| expression  | No       | string                                                 | Expression in specified language                           |
| reference   | No       | uri                                                    | Where the expression is found                              |