Noun

1. spectator, witness, viewer, watcher, looker, perceiver, percipient, observer, beholder

usage: a close observer; someone who looks at something (such as an exhibition of some kind); "the spectators applauded the performance"; "television viewers"; "sky watchers discovered a new star"

2. viewer, optical device

usage: an optical device for viewing photographic transparencies

See also: viewer (Dictionary)