c.barber said:
The correct meaning of this acronym is Entitlement Control Message.
This is a packet sent about every eight seconds by the provider to the smartcard. It contains the ECW (Encrypted Control Words - eight byte keys) which have to be decrypted by the card to get the DCWs. These Decrypted Control Words are then sent to the CSA (Common Scrambling Algorithm) register of the CAM (Conditional Access Module).
This, in turn, uses the DCWs as the CK (Common Key) to descramble the CSA scrambled video (and often audio) streams to give a clear picture (and sound). The CSA is now in the public domain.
However, as gameboy has already mentioned, ECM has by common usage also come to mean Electronic Counter Measure.
This is any tactic the providers use to make decrypting their ECWs more difficult. This can range from a simple key change to alterations in their algorithms.
I hope some find this basic explanation of common terms useful.