An activation subroutine is a standard set of procedures that must be followed prior to the initiation of a computer program including complex programs that control the powering up of a technological component.
Complex holographic programs such as The Doctor have extensive activation subroutines. Initially, the Doctor's activation subroutines could only be activated (and deactivated) by another member of the crew (something that lead him to state that he felt like a lightbulb). That was later changed and he could (de)activate himself when needed. (VOY: "Time and Again")