A convention is a set of agreed, stipulated or generally accepted social norms, norms, standards or criteria, often taking the form of a custom.
Certain types of rules or customs may become law, and regulatory legislation may be introduced to formalise or enforce the convention (e.g. laws which determine which side of the road vehicles must be driven). In a social context, a convention may retain the character of an "unwritten" law of custom (e.g. the manner in which people greet each other, such as by shaking each other's hands).
In physical sciences, numerical values (such as constants, quantities, or scales of measurement) are called conventional if they do not represent a measured property of nature, but originate in a convention, for example an average of many measurements, agreed between the scientists working with these values.

General

Customary or social conventions

Main articles: Norm (sociology) and Norm (philosophy) Social
There are generic conventions which are very closely tied to a particular artistic genre, and may even help to define what that genre is. Terms such as fan conventions and science fiction conventions could be interpreted in this manner, but more often refer to the meaning of "convention" as a gathering or the physical location of a gathering.
Other conventions that may simply be expectations are:

paintings are rectangular or square
stock devices (a comedy ends with a marriage, but a cowboy film can end with the hero riding off into the sunset)