Acronyms for Normally Open and Normally Closed. This refers alarm inputs/outputs and alarm devices. Most common alarm devices provide a contact closure to initiate the alarm. Some devices are normally an open circuit that closes a switch on alarm and others do the opposite. Most DVRs will accommodate either type with a menu selection.
N.O. (Normally Open) and N.C. (Normally Closed) describe alarm input states. N.O. circuits complete when triggered; N.C. circuits break. These terms define how sensors and alarms behave before and after an event.