These words refer to someone watching any sort of event. Spectator can indicate someone present at a sporting event or other happening; no direct participation is implied, but the word often suggests that a spectator has made the effort to attend or be a member of the audience: spectators at the tennis match; spectators lined up to watch the procession. Less often, the word can suggest the opposite: millions of people who unexpectedly became spectators to the world’s first televised murder. Onlooker , by contrast, more often suggests an accidental or chance viewing of some event: onlookers who happened to be present when the Ferris wheel collapsed. The word may also suggest someone who has deliberately withdrawn from events he might well have participated in: He chose to remain an onlooker during most of his family’s protracted quarrels.

Fan and barracker are more informal than the foregoing, but both specifically suggest the interested and voluntary viewing of something. Fan relates to spectator in particularly emphasizing the ardent advocacy of a given artist, performer or team: They performed in a hall that held thousands of screaming fans ; booed by fans for the home team. The word need not imply a physical gathering at all: a fan of the prolific mystery writer; a pre-sold market of fans for every new record they made. A barracker is the same as a fan , but, while a fan can be silent, a barracker is one who urges his man or team by loud shouts and cheering, as well as looking on: He was always a barracker for the winning team. In another verbal sense, it can signify a vocal attack: The crowd barracked the opening batsmen for slow play.

Peeping Tom and voyeur refer to onlookers who deliberately spy on others. Peeping Tom , the more informal of the two, may sometimes suggest devious attempt to gain information, but more often it suggests a mentally disturbed person who gets erotic pleasure from spying on unsuspecting people who are not fully dressed: a peeping Tom who kept watch on the bedrooms of the adjacent flats. Voyeur is the technical psychiatric term for such a person, although here the word includes, as well, any sort of erotic pleasure derived from looking rather than active involvement, even where stealth is not present: permissive parents whose open intimacies tend to make voyeurs of their children.

Observer is uniquely relevant to someone specifically assigned the role of watching rather than participating, particularly someone who remains impartial and has no authority to affect the outcome: U.N. observers deployed as members of the peace-keeping mission in the truce zone. The word can indicate also he role assumed by someone, such as a commentator or critic, who is a perceptive viewer of events and reports them to or analyses them for others: a keen observer of the modern art scene. In this case, it is the observer who may have the audience, whereas the things he reports may not. Witness may suggest accidental viewing, like onlooker : witness to the accident. It may also suggest someone who deliberately experiences something in order to report it: a crusading witness to racial injustice. It may, in fact, refer to the report itself; bearing false witness . The word, in the legal context, can refer either to onlookers or anyone else called to give evidence in court or to people called upon to observe and certify a transaction: witness who gave conflicting testimony; needing two witness to make the ceremony legal.