A data space is a relationship between trusted partners who adhere to the same standards and guidelines when it comes to data storage and sharing.

Another definition is:

• A data space is a decentralised infrastructure for trustworthy data sharing and exchange in data ecosystems, based on commonly agreed principles.

NB: The latter definition differs substantially from the former: relationship versus infrastructure.

• What are data spaces? Web page
• Positions paper Design Principles for Data Spaces. PDF