There is a very good account of what is meant by the term "ideology" in Don Luigi Giussani's book The Religious Sense (pp. 128-129 in my edition), a book which is a key text for the movement Communion and Liberation:
Ideology is a theoretical-practical construct developed from a preconception.
More precisely it is a theoretical-practical construction based on an aspect of reality, a true aspect, but taken up in such a way that it becomes unilaterally and tendentiously made into an absolute; and this comes about through a philosophy or a political project.
Ideology is built up on some starting point offered by our experience; this, experience itself is taken up as a pretext for an operation that is determined by extraneous or exorbitant preoccupations.
Pope Francis (who will be very familiar with Don Giussani's account of ideology - he presented the Spanish edition of The Religious Sense in Argentina before his election as Pope) has spoken more than once of "ideological colonisation". He adds to the understanding of "ideology" an understanding of the term "colonisation", which extends that concept beyond its geographical/political presentation, generally in the past, to an ideological presentation today, in the 20th and 21st centuries. Though he refers to ideological colonisation largely in the context of the family, his understanding of ideological colonisation includes the propaganda activities of the 20th century dictatorships, that is, colonisation by any ideology.
Let us be on guard against colonization by new ideologies. There are forms of ideological colonization which are out to destroy the family. They are not born of dreams, of prayers, of closeness to God or the mission which God gave us; they come from without, and for that reason I am saying that they are forms of colonization. Let’s not lose the freedom of the mission which God has given us, the mission of the family. Just as our peoples, at a certain moment of their history, were mature enough to say “no” to all forms of political colonization, so too in our families we need to be very wise, very shrewd, very strong, in order to say “no” to all attempts at an ideological colonization of our families. We need to ask Saint Joseph, the friend of the angel, to send us the inspiration to know when we can say “yes” and when we have to say “no”.
In the press conference on his way back to Rome, Pope Francis was asked to say more about this, and, in part, answered as follows, a response which clearly indicates that he considers gender theory as an "ideological colonisation of the family":
Ideological colonization. I’ll give just one example that I saw myself. Twenty years ago, in 1995, a minister of education asked for a large loan to build schools for the poor. They gave it to her on the condition that in the schools there would be a book for the children of a certain grade level. It was a school book, a well-thought-out book, didactically speaking, in which gender theory was taught. This woman needed the money but that was the condition. Clever woman, she said yes and made another book as well and gave both of them. And that’s how it happened. This is ideological colonization. They introduce an idea to the people that has nothing to do with the people. With groups of people yes, but not with the people. And they colonize the people with an idea which changes, or means to change, a mentality or a structure.
The promotion of gender theory, and of LGBT culture, to the whole of society is an example of an ideology - the move from respecting an aspect of life in society to making that aspect an absolute for the whole of life in society.
And is there not a possible new manifestation of an ideology in the conversation about climate change, where some are now referring to the effect of "humans" on the planet, rather than the effect of "people", as if the human race were just one species absolutely equivalent to other species?