·add. ·noun A tax in addition to the usual or normal tax; specif., in the United Kingdom, an income tax of sixpence for every pound in addition to the normal income tax of one shilling and twopence for every pound, imposed, by the Finance Act of 1909-1910 (c. 8, ss. 66, 72), on the amount by which the income of any person exceeds /3,000 when his total income exceeds /5,000.