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tr.v. de·rat·ed, de·rat·ing, de·rates
1. To lower the rating of (a device), especially because of a deterioration in efficiency or quality.
2. To lower the financial rating of (a corporation, for example).
(Government, Politics & Diplomacy) (tr) Brit to assess the value of (some types of property, such as agricultural land) at a lower rate than others for local taxation
Past participle: derated