Assumptions in general usage can be defined in a number of ways. Assumptions can refer to the act of possessing or asserting ownership on something, such as in the usage “assumption of ownership”. It can also be defined as the act of taking the responsibility of something upon oneself, such as in the use “assumption of a debt”. It can also refer to the act of taking for granted that something is in fact true. This is the general usage that most ties into the usage of assumptions in the project management sense. Assumptions in project management refer to specific factors that are considered to be true or certain when planning a project without necessarily having proof of it in reality. Assumptions, which are part of the progressive elaboration or a project, are critical to all aspects of the project planning process, despite the fact that even the most carefully considered ones typically carry with them a certain element of risk. It is imperative that the project team members who identify and document these assumptions attempt to minimize that risk whenever possible.
This term is defined in the 3rd and the 4th edition of the PMBOK.