What is a WBS Dictionary?
The WBS dictionary is the companion document to the work breakdown structure: for each WBS element it spells out the detailed description of work, acceptance criteria, responsible party, estimates, and dependencies. The WBS gives you the named boxes; the dictionary tells you exactly what's inside each one.
It's the antidote to the classic failure where two people read the same WBS label — "site preparation" — and picture two different jobs. If the box name is ambiguous, the dictionary entry is the referee.
Worked example
WBS element 2.3 "Site Preparation" could mean anything from mowing grass to moving a hillside. Its dictionary entry pins it down: clearing and grubbing of the 1.2-hectare footprint, cut-and-fill to the levels on drawing C-104, temporary drainage, and compaction tested to 95% Proctor; demolition of the old shed is excluded (it sits in 2.1). Owner: earthworks subcontractor; acceptance: survey verification against C-104. Now the estimator, the foreman, and the client all picture the same job.