Definition
DISCRETE FREQUENCY
A separate radio frequency for use in direct pilot‐controller communications in air traffic control which reduces frequency congestion by controlling the number of aircraft operating on a particular frequency at one time. Discrete frequencies are normally designated for each control sector in en route/terminal ATC facilities. Discrete frequencies are listed in the Chart Supplement U.S. and the DoD FLIP IFR En Route Supplement.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).