Generally understood as technical standards that are free from licensing restrictions. Can also be interpreted to mean standards that are developed in a vendor-neutral manner.

Sources: http://schoolofdata.org/handbook/appendix/glossary cited by EU OD; ODH

A standard developed or adopted by voluntary consensus standards bodies, both domestic and international. These standards include provisions requiring that owners of relevant intellectual property have agreed to make that intellectual property available on a non-discriminatory, royalty-free or reasonable royalty basis to all interested parties.

Source: US OD