Definitions of convention
a large meeting for a political party or group of people with similar interests
Many important people attend the annual Republican convention in the United States.
an agreement, usually signed by a large number of nations, which is governed by international laws
The Geneva Convention regulates the humane treatment of prisoners of war.
common usage or agreed practice, especially (in law) when referring to constitutional conventions. In common law jurisdictions, such conventions govern relations between the different branches of government in cases where there are no formal legal rules.
The King's wife is known as Queen by convention, but she may choose to be called the Princess Consort.