Phase
A phase is a major management segment of the project life cycle — analysis, design, build, transition — each holding a different character of work and its own deliverables, usually closed by a review gate.
The top layer of the tree
In a WBS, phases usually sit directly under the root. A typical systems-integration lineup: initiation, analysis, design, build, test, transition. Each phase holds a different kind of work and a different kind of deliverable — requirements specs from analysis, a live system and handover docs from transition.
Why it matters
Phase boundaries are decision points. At a phase gate you decide to proceed, rework, or stop. Staffing the build before design is approved means absorbing the full rework risk. Phases also set reporting resolution: executives track phases, working managers track work packages.
Common misconceptions
Having phases does not make a project waterfall — running iterations inside a phase is a common hybrid. The opposite error also appears: splitting phases by team name turns the WBS into an org chart. A phase partitions the life cycle and its deliverables, not departments.
wbsgantt templates ship WBS trees with standard IT phase structures, so you never design phases from a blank page.