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".
- 113.12. An effect that sets an object’s characteristic, or simply states a quality of that object, is different from an ability granted by an effect. When an object “gains” or “has” an ability, that ability can be removed by another effect. If an effect defines a characteristic of the object (“[permanent] is [characteristic value]”), it’s not granting an ability. (See rule 604.3.) Similarly, if an effect states a quality of that object (“[creature] can’t be blocked,” for example), it’s neither granting an ability nor setting a characteristic.
Example: Muraganda Petroglyphs reads, “Creatures with no abilities get +2/+2.” A Runeclaw Bear (a creature with no abilities) enchanted by an Aura that says “Enchanted creature has flying” would not get +2/+2. A Runeclaw Bear enchanted by an Aura that says “Enchanted creature is red” or “Enchanted creature can’t be blocked” would get +2/+2.