This paper introduces a formal concept of ideology and ideological system. The formalization takes ideologies and ideological systems to be situated in agent societies. An ideological system is defined as a system of operations able to create, maintain, and extinguish the ideologies adopted by the social groups of agent societies. The concepts of group ideology, ideological contradiction, ideological dominance, and ">dominant ideology of an agent society, are defined. An ideology-based concept of social group is introduced. Relations between the proposed formal concept of ideology and the classical concepts of ideology elaborated in social sciences are examined. A computational notation is presented, to support the realization of ideological systems in computationally implemented agent societies. The adequacy of the approach for the formal modeling and analysis of ideological issues is illustrated through three case studies.