According to the Business Rules Group, a Mission indicates the ongoing operational activity of the enterprise. The Mission describes what the business is or will be doing on a day-to-day basis.
The Mission statement should be focused on day-to-day operations, generic enough to cover all Strategies, and broad enough to cover the complete area of operations.
You can model Mission on the Enterprise Direction diagram, which supports the Business Motivation Model (BMM). You can also specify the Mission of your organization or project in the Enterprise Direction definitions.