In the context of ecclesiology, individualism is the notion (whether explicit or implicit) that the individual is his own highest ecclesial authority. The individualist does not submit or subordinate his interpretation to that of any other human on earth.
I have argued here repeatedly that there is no middle position between individualism and a recognition of sacramental magisterial authority. Now, one possible and somewhat common objection to my dilemma between individualism and sacramental magisterial authority is that my dilemma is a false dilemma, because those who submit to the magisterium of the Church are just as much individualists as are those who submit only to their own interpretations. The argumentation goes like this: Even those who submit to ecclesial authority must interpret the teaching of that ecclesial authority, and therefore those who submit to ecclesial authority are no less individualistic than are those who submit ultimately and only to their own interpretation of Scripture.
But that conclusion does not follow. The fact that each individual must interpret any form of communication does not entail that each individual is his own highest interpretive authority. Authority and interpretation are not the same thing. Therefore the fact that a person must interpret communication in order to understand it does not entail that such a person is his own highest authority, or is an individualist. What makes the individualist an individualist is not that he interprets Scripture, but that he treats himself as the highest interpretive authority for himself, as someone not under the interpretive authority of the Church. The person who submits his interpretation to the judgment of the magisterium of the Church must, of course, interpret the words in the magisterium's judgment, but being under the authority of the magisterium means that if necessary, he submits even his interpretation of the magisterium's judgment to the magisterium. He subordinates his interpretation of communication (whether in Scripture or in the teachings and judgments of the magisterium) to that of the magisterium. But the individualist does not subordinate his interpretations to that of the magisterium. So the necessity of interpretation in any communication does not entail that each person has equal interpretive authority or equal ecclesial authority. Nor does it entail that the individual is his own highest ecclesial or interpretive authority. Individualism should not be confused with the necessity of individual interpretation of communication. The latter does not imply or entail the former.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)