An observer is one who engages in observation or in watching an experiment.

Observer may also refer to:

Computer science and information theory

Fiction

Music

Physics

Politics and diplomacy

In quantum mechanics, "observation" is synonymous with quantum measurement and "observer" with a measurement apparatus and "observable" with what can be measured. Thus the quantum mechanical observer does not have to necessarily present or solve any problems over and above the (admittedly difficult) issue of measurement in quantum mechanics. The quantum mechanical observer is also intimately tied to the issue of observer effect.

A number of interpretations of quantum mechanics, notably "consciousness causes collapse", give the observer a special role, or place constraints on who or what can be an observer. For instance, Fritjof Capra writes:

"The crucial feature of atomic physics is that the human observer is not only necessary to observe the properties of an object, but is necessary even to define these properties. ... This can be illustrated with the simple case of a subatomic particle. When observing such a particle, one may choose to measure — among other quantities — the particle's position and its momentum"

The Observer is a newspaper for the residents of Northern Chautauqua County, NY and northwestern Cattaraugus County, NY, with offices located in Dunkirk, NY. Formerly known as the Evening Observer, and before then, the Dunkirk Evening Observer, it was originally delivered in the afternoon six days a week (Monday through Saturday), although it has since switched to morning delivery seven days a week.

The Observer was first published December 4, 1882 by founder Dr. Julien T. Williams. John D'Agostino, former news and managing editor, is the current Publisher.

The newspaper describes itself as a hometown paper, but it is owned by Ogden Newspapers Inc. of Wheeling, West Virginia; the paper is operated in a cluster along with The Post-Journal of Jamestown, New York and the Times-Observer of Warren, Pennsylvania.

On March 13, 2014, the entirety of the newspaper's Web site was placed behind a paywall. The site had been behind a paywall for most of the early 2000s but that paywall was eventually removed.