In the media I hear politicians say things such as "we have a mandate" and "the opposition has a mandate". What exactly does it mean when a particular party of government has a mandate?
In a democracy a politician can claim a 'mandate' if their policy has been concretely backed by the electorate - for example, if the policy is in the manifesto that the party stood on, successfully, for election. Another example could be a policy backed by referendum.
From the OED:
the authority to carry out a policy, regarded as given by the electorate to a party or candidate that wins an election: he called an election to seek a mandate for his policies