In the Magic: The Gathering Comprehensive Rules the term quality is used in context under its normal English meaning: it is a placeholder word that references some kind of attribute, or property of a card. There is no specific set of things about cards that is a "quality".
This is unlike words like "ability" and "characteristic". Those do have a specific definition in the rules.
Examples
- Protection from [quality]
- Hexproof from [quality]
- Splice onto [quality]
Rules
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