A rather comprehensive definition reads:

Ideologies are systems of ideas and values, which can be applied to various aspects of human life, such as politics, economy and world-views.

“Ideology” could also be related to science, but scientists would rather speak of hypotheses, which are established and valid as long as they cannot be proved untrue.

The literary scientist and philosopher Peter Tepe (“Fundamentalismus als Denkform”, University of Düsseldorf, 1999-2000) distinguishes positive and negative ideologies, using as parameters reason resp. verification.
Positive ideologies, which can be verified, show more tolerance versus deviant concepts than negative ones. The latter lean towards dogmatism, intolerance and fundamentalism.

In a more recent publication (“Ideologie”, in: “Grundthemen-Philosophie”, D. Birnbacher, P. Stekeler-Weithofer, H. Tetens (publisher), Berlin/Boston 2012) Tepe defines three sections of ideological theory:

• the cognitive, perceptive theory, which is basically a theory of perceptive errors, for instance caused by thinking adjusted to needs.
• the theory based on analysis of world-views
• the theory based on analysis of social or political concepts

World-views and political concepts should be advocated without dogma: This means tolerance versus alternative or competitive views and other political concepts.
The basic conditions for this attitude are safeguarded by an open society resp. by a liberal constitutional state, who can act powerfully but legally against all dogmatic systems, which are advocated by force.

The following definitions deal with the negative group of ideologies.

Jakob Barion says in his book “Was ist Ideologie? A study of the term and interpretation”, Bonn 1971, p. 7:

“Ideologies are opinions, which think of themselves as the only true ones, declare all other opinions as false and and hostile towards the truth and which must therefore be fought against.”  

Gottfried Thieme puts it even clearer (“Der ideologische Wahn, Über die Ursünde des Menschen”, Frankfurt/M. 1991, p. 24):

“An ideology is a systematic and intellectual development of an idea and/or of an ideal, which is presented with authority as true, even if it was not proved, cannot be proved or is already disproved by facts.”

The German/Jewish/American philosopher and writer Hannah Arendt gives in her foremost publication: “The Origins of Totalitarianism”, New York 1951, Frankfurt/M. 1955, Munich 1986, p. 719  the following description of the term ideology (translation of the German text):

” ……… the basic concept of an ideology is to develop out of an idea a premise, out of the perception of reality a precondition for events which should evidently and inevitably happen.”

If the reader has accepted the premise, f. i. the Nazi-theory of racial conflicts or the Marxism-theory of class struggles, the further steps will develop  ” ……. with a coherence which is never reached in reality ……. ” p. 723.

Terry Eagleton (“Ideology. An Introduction”, London/New York 2007, first edition 1991) outlines the sociological aspects of ideologies:

“Ideology is connected to the legitimacy of a dominant group or class” p. 5.

A similar argument is used by Louis Althusser (“Ideology and ideological State Apparatus”, Hamburg 2010).
First he cites the ideological term of Karl Marx:

“Ideology means the system of ideas and perceptions, which dominates the spirit of an individual or a sociological group”. p. 71.

Althusser’s conclusion:

“Ideologies support the hold on power of influential sociological groups at the expense of other non-privileged groups”. p. 92ff.

Jan Rehmann (“Introduction to Ideological Theory”, Hamburg 2008) describes the advantage of converging ideological theory and criticism by the examples of “Neoliberalism” resp. “Neoconservatism”. A theory primarily advocated by Milton Friedman and Friedrich A. Hayek, which pleads for deregulating markets and strengthening competition, against a welfare state and Keynesianism.
A preferable alternative would be the connection of competition (“free market”) with the social mitigation for people who are the losers in the free market game –> “social market economy”.