I'll define Martial Law as I saw it carried out under Katrina conditions:

1) No open possession of firearms
2) No movement after dark
3) No "groups" of citizens (more than 3) ever
4) etc, etc.

You can add to this: cops/soldiers/etc. can take from you whatever you have, for any reason. Any disobedience will be met with force and/or relocation (sans gear).

Basically, suspension of constitutional amendments--no one will argue your right to be "free" as long as you do what they say