An expert who is identified by a party as a potential witness. An expert is a person who, through education or experience, has developed skill or knowledge in a particular subject, so that he or she may form an opinion that will assist the fact-finder.

In Plain English:

In a personal injury case, the expert witnesses are usually doctors. They can testify about the plaintiff’s injuries because they have the education (medical school) and the experience (as a practicing doctor) to develop an expert opinion about the nature and extent of the injuries. The expert will be asked questions at a deposition, and if the case goes to trial, he or she will also testify there. The testimony that the expert witness gives can help the judge and jury decide the verdict of the case.