To turn a character into an exaggerated version of a single trait they originally held.
Named for Ned Flanders of The Simpsons fame, flanderization is the act of character derailment in the first degree. Where most characters, through character development, evolve into better defined and reasoned personalities, a character which is flanderized instead becomes a single defining feature and exaggerates that feature in every moment they hold the spotlight.
This tends to happen to characters which were once more important, but are seeing that importance wane as the work progresses. As the character is moved further and further away from the spotlight, only their most distinctive character traits are likely to be noticeable in the short amount of screentime (or pagetime, as the case may be) they are given. Writers might only remember the traits they wrote most recently when the character makes their next appearance, leading to a snowballing effect of flanderization. Alternatively, in a work with multiple writers, a character might only be known to a new writer by a few traits as the writer hadn't seen the character's more defined past states.
Flanderization can also occur to main characters who stay main characters. As JD points out, Superman, Batman, and Fonzie were all heavily flanderized in their own series. It's worth mentioning that each of those is an example of the multiple-creator situation.
This also isn't neccesarily a bad thing. Characters might be able to transition from main players to comic reliefs without much of a hitch due to losing most of their traits. Also, in the case of a character which is shifting from being a main focus of a work to a second-stringer, being too complex might actually detract from the work. While it's great for a character who only exists on one page of a novel to have complex motives and thought structures, if the amount of story space they take up is too large in comparison to their importance the narrative as a whole will suffer.
That said, this tends to be a sign of a poorly written work and should be avoided if possible. Characters are better served through development than derailment, and if you have a need for a new character type it might be a sign that you should introduce a new character.