# What is the Expression Constraint Language (ECL)

The *Expression Constraint Language* is a formal syntax for representing SNOMED CT expression constraints. Expression constraints are computable rules used to define a bounded sets of clinical meanings represented by either precoordinated or postcoordinated expressions. Expression constraints can be used to restrict the valid values for a data element in an EHR, as the intensional definition of a concept-based reference set, as a machine processable query that identifies a set of matching expressions, or as a constraint that restricts the range of an attribute defined in the SNOMED CT concept model.

The [Expression Constraint Language - Specification and Guide](https://app.gitbook.com/s/sOJBD7YbxAy9bD1Ko9L9/) has full details on ECL, including use cases, requirements, logical model, syntax specification, examples, and implementation considerations.

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