What is a 'convention' in UK politics?

Conventions are one of the sources of the UK constitution. They are effectively unwritten rules which regulate the conduct of government. An example is the Salisbury Convention, under which the House of Lords will not oppose any government legislation promised in the Commons majority party's manifesto. Conventions tend to develop gradually, unlike statute law which is created by Parliament. For example the convention of monarchs always granting Royal Assent to legislation passed by both Houses of Parliament developed gradually over the centuries.