To write something off in taxes means to claim a business expense on your end-of-year tax forms. The type of business expense you can deduct depends on the type of small business you run and what field you work in. For the IRS’s purposes, your business expense has to be ordinary and necessary to qualify as a write-off, and not every business expense is tax deductible. A tax write-off is different from an accounting or business write-off, which refers to the process of removing an asset from your books when it loses value.