An ideology is a collection of ideas. Typically, each ideology contains certain ideas on what it considers to be the best form of government (e.g. democracy or autocracy) and the best economic system (e.g. capitalism or socialism). The same word is sometimes used to identify both an ideology and one of its main ideas. For instance, socialism may refer to an economic system, or it may refer to an ideology which supports that economic system. The same term may also be used to refer to multiple ideologies and that is why political scientists try to find consensus definitions for these terms. For instance, while the terms have been conflated at times communism has come in common parlance and in academics to refer to Soviet-type regimes and Marxist–Leninist ideologies whereas socialism has come to refer to a wider range of differing ideologies distinct from Marxism–Leninism.  Political ideology is a term fraught with problems, having been called "the most elusive concept in the whole of social science". However, ideologies tend to identify themselves by their position on the political spectrum (such as the left, the centre or the right), though this is very often controversial. Finally, ideologies can be distinguished from political strategies (e.g. populism as it is commonly defined) and from single issues around which a party may be built (e.g. opposition to European integration or the legalization of marijuana), although either of these may be central to a particular ideology. There are several studies that show that political ideology is heritable within families.
The following list is strictly alphabetical and attempts to divide the ideologies found in practical political life into a number of groups and each group contains ideologies that are related to each other. The headers refer to names of the best-known ideologies in each group. The names of the headers do not necessarily imply some hierarchical order or that one ideology evolved out of the other. They are merely noting that the ideologies in question are practically, historically and ideologically related to each other. One ideology can belong to several groups and there is sometimes considerable overlap between related ideologies. The meaning of a political label can also differ between countries and political parties often subscribe to a combination of ideologies.