forcing function

Short definition from Wiktionary

Noun

1. 1
   A function in a system of differential equations that is only a function of time, unaffected by the other variables; or, more generally, any non-homogeneous source function in any variable.
2. 2
   An aspect of interface design that restricts a user's abilities in order to streamline completion of a task.
3. 3
   Any task, activity or event that forces one to take action and produce a result; impetus, impulse.