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.
@gameboy You are not wrong. In this case the ECM was a Electronic Counter Measure (change of RSA keys).
However, as I have stated there are two uses of this acronym in satellite TV. If you do a Google search on "ECM DVB" all references shown on the first page (of 34,600 hits - I didn't check them all) define ECM as Entitlement Control Message.
It should be fairly obvious by the context what ECM stands for in each case. Unfortunately, both these uses of ECM are in the field of satellite TV encryption, so to the novice there could be some confusion. My post was just to explain the difference.
We could also talk about External Cardiac Massage - but that is another subject! :-))