To externalize is to take inner feelings and put them outside you — to show them, say them, write them, act them — so they are not only in your head. One person externalizes anger by talking; another by slamming a door; a painter may externalize sadness in a picture. That is the core: inner life given an outer shape. Though actually, psychology uses a second, slightly different sense: to treat the cause of a problem as something outside yourself (“It’s the world’s fault”). That can be honest, or it can be a way of not looking in. The word is a bit formal. British spelling is often externalise. People stretch it in offices too (“externalize the cost”), but that is a different, money-and-systems use; the sense here is feelings made outer.

外化；把內心感受表現出來