MicrosoftВ® Office Project 2007 Step by Step (Step By Step (Microsoft))

The condition of resources when they are assigned to do less work than is their normal work capacity. For example, a full-time resource who has only 25 hours of work assigned in a 40-hour work week is underallocated.

A standard way of measuring the capacity of a resource to work when you assign the resource to a task in Project. Units are one variable in the scheduling formula: Duration×Units=Work.