In syntax, dislocation refers to a sentence structure in which a constituent occurs outside of its usual boundaries. The dislocated constituent is often accompanied by a pause in speech and a comma in writing. Two types of dislocation occur: left dislocation and right dislocation.
In the sentence "John A. MacDonald, he was the first prime minister," we have an example of left dislocation. The meaning is the same as in the sentence "The first prime minister was John A. MacDonald," but in the first sentence emphasis is given to John A. MacDonald. In left dislocation the topic is advanced. It occurs earlier in the sentence than is usually the case.
With right dislocation, the constituent is postponed. Consider the following sentence: She won the election, Angela Merkel. She won the election is a good sentence, but Angela Merkel is added to clarify who the subject of the sentence is.
Dislocation is very common in language. Left dislocation serves to emphasize the topic of the sentence and right dislocation provides clarification of the subject. The term dislocation is used because the constituent occurs outside of the regular clause boundaries.