In What are the qualifications to be an expert witness?, David Goodwyne writes:
What an exactly is an expert witness? An expert witness, as the name suggests, is an individual who is a person whose opinion can be relied upon in respect of an issue or fact that is relevant to the scope of his expertise. Not anyone can be an expert witness as this person is usually an individual with the right level of training, education and experience so that he is deemed an “expert”. Just being “competent” or “good” at your job doesn’t necessarily qualify you to become an expert.
An expert witness’ role is crucial in a trial when there is, for example circumstantial evidence or issues that cannot be directly proven. Hence, the evidence of the expert witness is important because it will help convince the judge or jury of the case theory that the attorney intends to potray. Common examples of expert witneses include forensic scientists (eg: to explain things like estimated cause of death in the event this is cannot be directly and obviously ascertained), psychiatrists or psychologists (eg: to explain a defendant’s state of mind, particularly in cases where insanity or diminished responsibility is claimed as a defence), and engineers (to explain technical concepts in the event this is a core issue of a trial).
Read more: helium.com.