Typically, an expert witness is someone whose level of scholarship, knowledge, or professional experience in a specific or specialized field allows them to provide testimony—their “expert opinion”—before a court of law on the evidence or circumstances of a criminal or civil case. A court must recognize this person as a qualified “expert witness” before they can testify as such.
Less formally, an expert witness’s qualifications will usually include previous experience as an expert witness, the ability to communicate complex ideas clearly, and the capacity to justify their expert opinion in the circumstances and evidence of the case in combination with their own experience in their specialized field.