In common law jurisdictions, the generic term officer of the court is applied to all those who, in some degree in the function of their professional or similar qualifications, have a part in the legal system.
They are tasked to participate to the best of their ability in the functioning of the judicial system to forge justice out of the application of the law and the simultaneous pursuit of the legitimate interests of all parties and the general good of society.
Foremost those who make the decisions that determine the course of justice and its outcome: These are people who may appear in court and testify or offer opinions due to their expertise or experience in a given subject.
Their opinions sometimes rise to the level of scientific evidence and are evaluated by judges and juries to reach conclusions or verdicts.
These are people whose professional duties are important to the functioning of the court system.