a number of people or a particular group of people who watch, read or listen to the same thing:
An audience of millions watched the wedding on TV; TV/cinema/movie audiences; His book reached an even wider audience when it was made into a movie; The target audience for this advertisement was mainly teenagers.
These are all words for a person who sees something happen.
witness a person who sees something happen and is able to describe it to other people; a person who gives evidence in a court of law: Police have appealed for witnessesto the accident. audience a group of people who have gathered to watch or listen to something such as a play or concert; the number of people who watch or listen to the same thing on television, the radio or in the cinema. viewer a person watching television; a person who looks at something: The programme attracted millions of viewers; Some of her art is intended to shock the viewer. observer a person who sees something happen: According to observers, the plane exploded shortly after take-off. spectator a person who is watching an event, especially a sports event: The stadium holds 75000 spectators. onlooker a person who watches something that is happening but is not involved in it: A crowd of onlookers gathered at the scene of the crash. passer-by a person who is going past somebody/something by chance, especially when something unexpected happens: Police asked passers-by if they had witnessed the accident. bystander a person who is near and can see what is happening when something such as an accident or fight takes place: Three innocent bystanders were killed in the crossfire. eyewitness a person who has seen a crime or accident and can describe it afterwards.
PATTERNS AND COLLOCATIONS • before/in front of a(n) witness/audience/spectator/ onlooker/passer-by/bystander/eyewitness • a television/TV audience/viewer • a cinema audience/viewer • to attract/draw/pull in (an) audience/viewers/ spectators • a(n) audience/viewer/observer/spectator/onlooker/ passer-by/bystander witnesses something