A type of tax allowance used in the UK based upon the amount of fixed assets used in the business. These WDAs reduce the tax bills of companies that would otherwise be liable to taxation on their profits.
Not as dull as you would first expect! Businesses that spend money on expenses - travel, salaries, rent etc - deduct all of this cost against their profitability and their tax bill. Where fixed assets are bought for use in a business, they have a life of many years. This allowance is the proportion of the original cost of the asset that can be deducted from profitability for tax calculations.