Here is how Florida law defines it: Under Florida Statute 874.03 (1): "Criminal gang" means a formal or informal ongoing organization, association, or group that has as one of its primary activities the commission of criminal or delinquent acts, and that consists of three (3) or more persons who have a common name or common identifying signs, colors, or symbols, including, but not limited to, terrorist organizations and hate
groups.
Also included in those statutes are the criteria for an individual to be documented as a gang member or associate, and once you are, if you are caught committing gang-related criminal activity (also defined in the stautes), enhanced penalties can be sought by prosecutors for those crimes. There's a variety of ways outlined in the law to determine relationship with a gang, but it's often self-admitted. And many feel the need to sport some ink for their crew as well as the typical colors and tagging; it is not something they try to hide. In their world, it is important to be affiliated and instantly recognized for that affilation - kind of like it is in a lot of our worlds. The difference is most clubs, fraternal organizations, churches and prep schools aren't making criminal activity one of their primary activities. Secondary, perhaps.
Note the statute doesn't just include street gangs, but also terrorist organizations and hate groups. Some of those likely make up the reported total of 47.