Nonconformance is to be used when you perform quality control and the result is “not OK”. Nonconformance is about products and services, that is why clause 8.7 is “Control of nonconforming outputs”. The outputs are the products or services.
Nonconformity in clause 10.2 is to be used when you perform monitoring and measuring and the result is “not OK”. For example, when you evaluate the process “Manufacture product” and you realize that the rate of defects is above the maximum tolerated you have a nonconformity. This time the problem is not with the product but with process performance.
During audits, auditors only raise nonconformities even when it is about a particular product. For example, an auditor can raise a nonconformity because the nonconforming product is not correctly segregated. The concern of the auditor is not the product it is the system not working according to standard or procedure.
The following material will provide you with information about nonconforming and nonconformities:
You are right ISO 9000:2015 does not provide a specific definition for the term "nonconformance" in its standards. However, it uses related terms like "nonconformity" and "conformity" in ISO 9001 and other quality management standards.
While these terms are often used interchangeably, a nonconformity is a more precise term and is the one formally used in ISO 9001 to describe instances of noncompliance with the standard's requirements. Nonconformance, on the other hand, is a broader term that can encompass various types of failures to meet quality criteria. Many times people use them to differentiate a nonconformity according to clause 8.7 from a nonconformity according to clause 10.2.
If you search the word “nonconformance” in this site, you will see that it is used very commonly by those asking questions.