We monitor your
systems to keep
your business running...
Assumption [uh-suhmp-shuh n]
There may be external circumstances or events that must occur for the project to be successful (or that should happen to increase your chances of success). If you believe that the probability of the event occurring is acceptable, you could list it as an assumption.
An assumption has a probability between 0 and 100%. That is, it is not impossible that the event will occur (0%) and it is not a fact (100%). It is somewhere in between. Assumptions are important because they set the context in which the entire remainder of the project is defined.
If an assumption doesn't come through, the estimate and the rest of the project definition may no longer be valid.