An Execution Environment is a node that offers an execution environment for specific types of component that are deployed on it in the form of Executable Artifacts. This is depicted in a Deployment diagram.
Execution Environments can be nested; for example, a database Execution Environment can be nested in an operating system Execution Environment. Components of the appropriate type are then deployed to specific Execution Environment nodes.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.664) states:
An execution environment is a node that offers an execution environment for specific types of components that are deployed on it in the form of executable artifacts.
The OMG Unified Modeling Language specification, (v2.5.1, p.658) also states:
Typically, ExecutionEnvironments are assigned to some, often higher level, Device or general system Node via the composition relationship defined on Node. ExecutionEnvironments can be nested (for example, a database ExecutionEnvironment might be nested in an operating system ExecutionEnvironment). ExecutionEnvironment may have explicit interfaces for system level services that can be called by the deployed elements. In such cases, software ExecutionEnvironment services should be explicitly modeled. Application Components of the appropriate type are then deployed, with a Deployment relationship, to specific ExecutionEnvironment nodes or the Manifestations relationships of DeployedArtifacts. For each component Deployment, aspects of these services may be determined by properties in a DeploymentSpecification for a particular kind of ExecutionEnvironment.