For more definitions, see https://elsmar.com/level2/m-vs-m.html
A Registrar s Opening Meeting Outline V
Audit Process Continued
Major Nonconformance
The absence of, or the failure to implement and maintain, one or more required management system elements, or a situation which would, on the basis of available objective evidence, raise significant doubt as to the quality of what the registration customer is supplying. An assessment team may judge many minor nonconformities against a single quality system element to be a significant breakdown of a quality management system element.
Minor Nonconformance
Any other non-conformance and is normally easily corrected and verified.
Opportunity
Neither a major or minor non-conformance. It is used to document items that may help a customer improve.
ISO 19011 - Quality and Environmental Management Systems Auditing Forum Discussions