The Congressional Research Service defines sequestration this way:

"In general, sequestration entails the permanent cancellation of budgetary resources by a uniform percentage. Moreover, this uniform percentage reduction is applied to all programs, projects, and activities within a budget account.
However, the current sequestration procedures, as in previous iterations of such procedures, provide for exemptions and special rules. That is, certain programs and activities are exempt from sequestration, and certain other programs are governed by special rules regarding the application of a sequester.