These words all mean a seeking for information or truth. Inquiry is the most general term and is loosely applicable to all the others. An inquiry may range in importance from any request for information about ordinary matters to an investigation of an official nature: to make inquiries at a service station concerning road directions; an inquiry made by the Health Department about a spreading epidemic of measles; an official inquiry into the cause of the derailment. Enquiry is an alternative spelling of this word.

Examination is also a general term; but an examination is generally stricter or more formal than an inquiry , involving careful scrutiny or inspection: an examination of all the points in a formal argument; an examination of one’s motives for doing something. In law, an examination is an inquiry made by direct questioning or by the taking of evidence: The accused underwent examination by the Crown prosecutor.

An inquest is an inquiry or examination, but its meaning is now almost entirely restricted to that of a legal investigation. In its best-known sense, an inquest is an investigation by a coroner of the causes of a death when there is sufficient evidence to suspect that it occurred for other than natural reasons.

Inquisition and probe are both searching, official investigations of an individual or of a group, made in order to dig out facts, generally with a view to proving the existence of illegal acts or of heterodoxy. Inquisition is chiefly thought of in its historical meaning, that of the examination and punishment of heretics, as practised by the ecclesiastical courts of Europe during the late Middle Ages and the Reformation. The term also is applied to the body of church officials engaged in this activity. In its more general sense, inquisition implies persecution by means of relentless investigation, and harassment by persistent and prolonged interrogation. Probe is an Americanism when used in the sense of inquisition , but is becoming popular elsewhere, especially in newspaper usage. It denotes an investigation that goes deeply below the surface in a manner reminiscent of a surgeon’s instrument exploring a gunshot wound. A probe may be made either by a legislative body or by a committee set up within such a body, into alleged misconduct or illegal practices on a large scale: a probe into a tax evasion on the part of a large company; a probe into the misuse of public funds.

A study is an inquiry made by gathering information in some detail in order to arrive at certain conclusions or to obtain a body of specialized knowledge: a study of the habits of the mountain gorilla; a study of the sluggish currents of the Sargasso Sea. Studies are also conducted to gain an understanding of the causes of an undesirable social condition so that improvements may be made: a study of the traffic situation in a large city; a study of the incidence of relapse in mental illness.