I found a nice definition of Planning that fits well with the Five Immutable Principles of project success.
deciding what action to take
and allocating the necessary resources
in order to
achieve the objective,
meet the wishes of all those involved, and
be as efficient as possible.
The Plan is the strategy for successfully delivering the capabilities needed to fulfill the mission of the project or program. We need the Plan to recognize what DONE looks like in Measures of Effectiveness. The Measures of Effectiveness are operational measures of success that are closely related to the achievements of the mission or operational objectives evaluated in the operational environment, under a specific set of conditions.