In a multi-party democracy, a coalition government is one in which two or more political parties agree to share power. Coalitions may be formed in times of national crisis or political turmoil, though they are the norm in most European countries and are common in Japan even when one party has a working majority.
Coalition governments are most characteristic of parliamentary political systems (e.g. the Westminster system). Typically, the head of state will be outside the government and will - perhaps only ceremonially - approve the coalition, to be led by the head of government. A coalition need not include the party that received the most votes, as long as the coalition puts together a sufficient number of votes to be recognised. This is the current situation in the State of Israel.
Where two parties dominate the political system, such as in the United States of America or, until 2010, the United Kingdom, coalitions are rarely formed because elections typically result in a government with a majority of seats in the chamber and an opposition party with enough representation to form an effective counterweight to it. An exception is Japan, where the historical concentration of power in a relatively small proportion of the population has led in part to a lack of major policy differences between mainstream parties or factions; combined with the differing fortunes of parties in both houses of parliament, this has often resulted in a drive to form coalitions even when the winning party has a firm majority. For example, the Democratic Party of Japan (DPJ) formed a coalition government with the People's New Party and the Social Democratic Party in 2009, partly because the DPJ lacked a firm majority in the upper house; however, the alliance with the SDP lasted less than a year.