Input is a term that refers specifically to any particular item, product, or mechanism that refers to an internal or external device that can be used for the purposes of triggering the progression of a particular process. This form of input is always a requirement before the process is actually able to proceed as such. Input can come in many different means such as such as specific types of data that can be entered into the system, the results of which can trigger the process to begin again. Input can be derived in many cases as a resultant output from a previous work schedule activity or a previous work process. Input can be the result of a compilation of various phases of a particular project, it could be the result of one specific work activity, or it can be data that is entirely independent of the overall project processes but no less essential to the continuing progress of the overall activity.
This term is defined in the 3rd and the 4th edition of the PMBOK.