An expert is a professional with technical, scientific, or artistic knowledge that is important in helping to better understand what happened.
For example, it may be necessary for a doctor to explain the wounds suffered by the victim and how they were inflicted.
Some examples of experts who can collaborate in the process are doctors, psychologists, psychiatrists, computer engineers, etc.
The expert writes a report, which includes their conclusions on what they examined and assessed. If summoned to appear at the trial, the expert testifies and answers questions about the examinations and assessments they conducted and on their findings.