A government is an organization that attempts to maintain control of a territory, known as a state. ("State" may also be used to refer to the government itself.) "Control" may involve actions such as collecting taxes, controlling entry and exit to the state, preventing encroachment of territory by neighbouring states and preventing the establishment of alternative governments within the state.
Governments use a variety of methods to maintain control, such as military and police forces (particular under despotism, see also police state), making agreements with other states and maintaining support within the state. Typical methods of maintaining support include providing justice and social welfare, claming support of deities, providing benefits to influential groups, holding elections for important posts within the state, limiting the power of the state through laws and constitutions and appealing to nationalism.
A previous definition, with useful links:
The governing body of a country or subnational entity; generally a group of politically appointed officials (often called ministers or secretaries) who in theory act as advisors to the head of state, though they are generally themselves the heads of various offices or departments which give them a certain amount of direct power in specific areas. In most modern democracies, the elected legislative assembly has the power to dismiss the government, though the head of state generally has great latitude in appointing a new one. A government is sometimes also called a cabinet.