A qualified specialist who gives opinion testimony to help a judge or jury understand disputed facts.

"Qualified" means the person has education, training, experience, or skill beyond what an ordinary witness has. Unlike a fact witness, who tells what they personally saw or heard, an expert witness may draw conclusions from records, tests, measurements, or professional knowledge. "Opinion testimony" means the witness can explain not just what happened, but what it likely means - for example, whether a crash caused a spinal injury, whether a worksite was unsafe, or what future treatment may cost. And "help" is the key limit: the opinion must assist the court, not simply repeat common sense or argue the case.

In practice, expert witnesses often shape whether a claim is taken seriously, settled, or tried. Doctors, accident reconstruction engineers, vocational experts, and economists are common examples. Their opinions can support causation, prove damages, or challenge the other side's version of events.

In New York, expert proof can be decisive in injury cases governed by Insurance Law § 5102(d), the state's serious injury requirement under the no-fault system. A treating doctor or retained medical expert may be needed to connect MRI findings, range-of-motion limits, or long-term disability to the accident. Courts also apply rules on expert disclosure under CPLR 3101(d), which can affect whether an expert gets to testify at all.