This can have a lot of meanings by a lot of good folks, but my definition is as follows:

A Master Carver is one that has learn the craft well and completes a fish decoy that properly swims and attracts fish. It may or may not be attractive and may be just down right ugly. The carver is considered by his peers as a Master Carver. Does not need to have won contest to be considered as a Master Carver. Normally Master Carver's decoys are so well made that they are highly sought after. Because there is not a certification for a Master Fish Decoy Carver - a definition is very hard to state. My final words are - if you like the work and you consider the carver that good to meet your standards of what a Master Fish Decoy Carver is, then so be it - to you he/she is a Master Fish Decoy Carver.