US Federation of Worker Cooperatives definition:
Worker cooperatives are values-driven businesses that put worker and community benefit at the core of their purpose. In contrast to traditional companies, worker members at worker cooperatives participate in the profits, oversight, and often management of the enterprise using democratic practices. The model has proven to be an effective tool for creating and maintaining sustainable, dignified jobs; generating wealth; improving the quality of life of workers; and promoting community and local economic development, particularly for people who lack access to business ownership or sustainable work options.
A longer definition from this group is downloadable here.