Duplex - This is the term given when you have to switch between frequencies.  You have to do this          because the base transmits on a different frequency to the one that it receives on and vise versa for the    mobile.

e.g  Northumbria Police -  Base     -  152.0875

                                        Mobile  -  143.0750

Simplex - This is the term given when two different sets communicate on the same frequency.

e.g Cramlington Police - 451.8250 NFM ( the base and mobiles can be heard here )

MASC - Stands for Marconi Advanced Scrambler.  This is the encryption system used by the police on the 450-453 Mhz band.  It can be used in Key Code 1, 2 or Clear Speech ( You are able to hear when in clear speech.  There is now a page dedicated to MASC available on this site.  Click Here.

ATIS - Automatic Terminal Infomation Service.  This is active at busy airports which tells incoming and overflying aircraft of the conditions at the airport e.g Wind Speed and Precipitation.

UHF - 'Ultra High Frequency'  400Mhz +

VHF - 'Very High Frequency'   140Mhz +

AM -   ' Amplitude Modulation'

FM  -    'Frequency Modulation'

PROMA - Professional Radio Operates Monitoring Association.  Founder is Paul Wey.

OFFSET - This is the function on many scanners that lets you listen to DUPLEX communications.  You programme in the frequency distance between the base and the mobile and while pressing a button you can switch between the two frequencies.

More terms will be added when necessary.