The system of preventing unauthorised access to the resources of an IT product, programs, processes, systems, or other IT products. Some suppliers consider preventing unauthorised users from logging on to the system to be access control. In reality, access control should also stop logged on users accessing objects (files, devices, etc) for which they have no authorisation.

an inter-connected group of components that work to regulate and monitor building access by electronically controlling doors and elevators

A type of security system that regulates entrance to an area using a keypad or cardreader. This might be as simple as a code enterd on a keypad to open a door or a more complex application such as controlling access to floors in an elevator.

The mechanism, which takes care of the conditional access.