Each PMI process has a defined set of inputs, tools and techniques, and outputs. The PMBOK glossary defines input as "any item, whether internal or external to the project that is required by a process before that process proceeds. May be an output from a predecessor process."
An example of an external input is a market force that creates the need for the project. Another external input would be a government inspection that must be performed before a project deliverable can be signed-off as complete. An example of an internal input is the project charter which states at a high-level the goals and constraints of the project. The charter will then feed into other processes that follow those guidelines or expand upon them.
Some inputs:
activity cost estimates, activity duration estimates,
cost management plan,
enterprise environmental factors,
human resource management plan,
make-or-buy decisions,
organizational process assets,
procurement documents, procurement management plan, procurement statement of work,
project calendars, project documents, project funding requirements, project management plan, project schedule,
quality management plan,
requirements documentation,
risk management plan, risk register,
schedule data, schedule management plan,
scope baseline, seller proposals, source selection criteria, stakeholder register,
work performance data