Usually

A convention is a set of , stipulated, or generally accepted , norms, social norms, or , often taking the form of a custom.

Certain types of rules or customs may become law and regulatory legislation may be introduced to formalize or enforce the convention (for example, laws that define on 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 (for example, 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.

A convention is a selection from among two or more alternatives, where the rule or alternative is agreed upon among participants. Often the word refers to unwritten customs shared throughout a community. For instance, it is conventional in many societies that strangers being introduced shake hands. Some conventions are explicitly legislated; for example, it is conventional in the United States and in Germany that motorists drive on the right side of the road, whereas in New Zealand and the United Kingdom motorists drive on the left. The standardization of time is a human convention based on the solar cycle or calendar. The extent to which justice is conventional (as opposed to natural or objective) is historically an important debate among philosophers.

...
Wikipedia