A CQL library is text document (.cql, .xml, and .json) that contains CQL expressions, definitions, functions, and other declarations that can be used across measures. Each eCQM contains a primary library that defines the criteria used by the populations of the measure. The Health Quality Measure Format document references this library and defines the populations by identifying which expressions in the CQL library define each population (e.g., Inpatient Encounter, terminologies). Measures may include references to a shared library, which can be either referenced throughout a single measure or across different measures (and even CDS rules) to share definitions and functions like "Hospitalization". Library sharing minimizes efforts with measure development and implementation.
For more information on libraries, refer to the Using Libraries to Shared Logic section of the CQL Implementation Guide.