What is Bracket Creep?

Legal Definition
Bracket creep describes the process by which inflation pushes wages and salaries into higher tax brackets, leading to a fiscal drag situation.

Most progressive tax systems are not adjusted for inflation. As wages and salaries rise in nominal terms under the influence of inflation they become more highly taxed, even though in real terms the value of the wages and salaries has not increased at all. The net effect is that in real terms taxes rise unless the tax rates or brackets are adjusted to compensate.
-- Wikipedia
Legal Definition
Movement into a higher tax bracket as taxable income increases.