Internalization has different definitions depending on the field that the term is used in. Internalization is the opposite of externalization

Externalization

Externalization means to put something outside of its original borders, especially to put a human function outside of the human body. The opposite of externalization is internalization....

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Generally, internalization is the long-term process of consolidating and embedding one’s own beliefs, attitudes, and values, when it comes to moral behavior. The accomplishment of this may involve the deliberate use of psychoanalytical or behavioral methods.

When changing moral behavior, one is said to be "internalized" when a new set of beliefs, attitudes, and values replaces or habituates the desired behavior.