There are a lot of 'definitions' of quality. You could search the forums on this topic, and you'll find some of them.
To respond to your specific question, I suppose that an interviewer doesn't expect a definition from you, he/she whould expect your understanding of quality, which you'll have to explain.
For example, if you'd define quality as adherence to requirements and standards or as degree of customer's satisfaction, he/she could ask you why, how to measure, to give examples,...
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