node.md

In CircleCI, the hosts/executors on which a pipeline is executed, is called a runner.
You reference a runner on a job in the file .circleci/config.yml like this:

version: 2.1
workflows:
  testing:
    jobs:
      - runner
jobs:
  runner:
    machine: true
    resource_class: your-namespace/your-resource
    steps:
      - run: echo "Hi I'm on Runners!"

The above example is taken from CircleCI docs