Glossary
Expert witness
A witness with specialized training, education, or experience who's allowed to give opinions on technical subjects beyond ordinary knowledge. Examples: a doctor explaining injuries, an accountant explaining financial damages, an engineer explaining a product defect.
Expert witnesses are different from regular (fact) witnesses in two big ways. First, they can give opinions, not just describe what they saw. Second, they have to be qualified by the court: both sides can challenge whether the expert has enough credentials and whether their methodology is reliable.
Experts are usually paid for their time. Both sides typically hire their own experts in technical cases, which is why a complex case can come down to a battle of competing experts trying to convince the jury which methodology is sounder.