Noun

1. A rule of custom established by general usage
   
   Example: An unwritten law of gentlemanly decorum
   
   
2. Law which rests on custom by judicial decision, and not on a written command, decree or statute
3. A custom in some communities granting a measure of immunity to those who commit criminal acts of revenge in support of personal or family honor, especially in cases of seduction, adultery, etc.

See also

• common law