Luminosity is a measure of how many collisions are delivered to and recorded by the detector.
Instantaneous luminosity is usually expressed as the number of collisions per square centimeter per second.
Integrated luminosity is the integral of instantaneous luminosity over time and is a measurement of data size. It is usually expressed in units of inverse cross section.
Knowing the luminosity is important to determining and measuring accelerator and detector performance and operation. It is also an essential component for measuring cross sections and for setting limits on beyond-SM processes.
Measurement of luminosity is done with several systems in the detector.