A definition of characterization is as follows (taken from google): A description of qualities or peculiarities: a list of places of interest, with brief characterizations of each. Representation of a character or characters on the stage or in writing, especially by imitating or describing actions, gestures, or speeches.
Not very informative, especially as it even includes the word we are defining in the definition itself.
Here’s a better one: Characterization is the way in which authors convey information about their characters. Characterization can be direct, as when an author tells readers what a character is like (e.g. “George was cunning and greedy.”) or indirect, as when an author shows what a character is like by portraying his or her actions, speech, or thoughts (eg. “On the crowded subway, George slipped his hand into the man’s coat pocket and withdrew the wallet, undetected.”). Descriptions of a character’s appearance, behavior, interests, way of speaking, and other mannerisms are all part of characterization. For stories written in the first-person point of view, the narrator’s voice, or way of telling the story, is essential to his or her characterization.
This was taken from Narrative Elements Characterization
There are several steps to making and keeping your character ‘in character’. One is to create them, another is to build them, and so on.