In literature, as in life, we often see growth, change, and internal conflict carried out in a single character. The term one-dimensional character in a book review refers to a character who lacks depth and who never seems to learn or grow.
If a character is one-dimensional, he or she does not demonstrate a sense of learning in the course of a story. Authors may use such a character to highlight a certain trait; usually an undesirable one.
A one-dimensional character can be summed up in a certain trait or characteristic. In All Quiet on the Western Front, Paul Bäumer's high school teacher, Kantorek, is an example of a one-dimensional character, because he maintains a sense of idealistic patriotism, despite his encounters with war atrocities.