Refers to a person's admirable trait or a thing's positive attribute. Somewhat formal and often used in written or evaluative contexts.
彼の最大の美質は誠実さだ。
His greatest virtue is his sincerity.
この計画の美質は、その柔軟性にある。
The good quality of this plan lies in its flexibility.
Compound of 美 (beauty, virtue) and 質 (quality, nature). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning of an admirable quality.