In production theory, an input is a factor or resource which, when combined with other factors of production help to produce a given level of output.
Some of these inputs are outputs of other firms.
For example flour is an output of flourmill. It is however the import of Alalo’s Fresh one bakery for the making of bread and meat pies.
Input may be fixed or variable. A fixed input is one whose quantity does not change in response to desired change in output whilst a variable input is one whose quantity changes in with the desired output