An anachronism (from Greek ana, back, and chronos, time) is something that is out of its natural time or appears to be. For example, if a play set during the Roman Republic portrays a person using a Computer, the computer is an anachronism.
However, an apparent anachronism may reflect our ignorance rather than a genuine chronological Anomaly. A popular view of History presents an unfolding of the past in which humanity has a primitive start and progresses toward development of technology. Alleged anachronistic artifacts demonstrate contradictions to this idea. Some archaeologists believe that seeing these artifacts as anachronisms underestimates the technology and creativity available to people at the time, although others believe that these are evidence of alternate or "fringe" timelines of human history.
If one envisions human technological advancement as being roughly parallel to the expansion and decline of human civilizations -- that is, progressing in a "three steps forward, two steps back" sort of manner -- then at least some (perhaps even many) apparent "anachronisms" are to be expected. A good example of this would be Concrete, being used in the past by various ancient cultures only to be forgotten about and then re-invented at a later time by another culture, until the present, at which point the technology is employed globally and unlikely to slip into obscurity again.
In particular, the artists, on the stage and on the canvas, in story and in song, assimilated their characters to their own nationality and their own time. Roman soldiers appear in Renaissance military garb. The Virgin Mary was represented in Italian works with Italian characteristics, and in Flemish works with Flemish ones (this still occurs today, as in the example of the Black Madonna). Alexander the Great appeared on the French stage in the full costume of Louis XIV of France down to the time of Voltaire; and in England the contemporaries of Joseph Addison found unremarkable (in Pope's words)
However, in many works, such anachronisms are not simply the result of ignorance, which would have been corrected had the artist simply had more historical knowledge. Renaissance painters, for example, were well aware of the differences in costume between ancient times and their own, given the renewed attention to ancient art in their time, but often chose to depict ancient scenes in contemporary guise. Rather, these anachronisms reflect a difference of emphasis from the 19th and 20th century attention to depicting details of former times as they "actually" were. Artists and writers of earlier times were usually more concerned with other aspects of the composition, and the fact that the events depicted took place long in the past was secondary. Such a large number of differences of detail required by historic realism would have been a distraction.
By contrast, in recent times, the progress of archaeological research and the more scientific spirit of history have encouraged audiences and artists to view anachronism as an offense or mistake.
Dramatic productions sometimes use anachronism for effect. In particular, directors of Shakespeare's plays may use costumes and props, not only of Shakespeare's day or their own, but of any era in between, or of an imagined future: the musical Return to the Forbidden Planet crosses The Tempest with popular music to create a science fiction musical. A similar approach was used in the 2001 film Moulin Rouge!, in which a diverse selection of 20th-century music is used over a Fin de siècle backdrop. Other films, such as A Series of Unfortunate Events or Richard III may create worlds so full of various conflicting anachronisms as to create a unique stylistic environment that lacks a specific time period setting. This use of stylistic anachronism is often done in children's movies.
Comedic works of fiction set in the past may use anachronism for a humorous effect. One of the first major films to use anachronism was Buster Keaton's The Three Ages, which included the investion of Stone Age Baseball and modern traffic problems in classical Rome. Mel Brooks' 1974 film Blazing Saddles, set in the Wild West in 1875, contains many blatant anachronisms from the 1970s, including a stylish Gucci costume for the sheriff, an automobile, a scene at Grauman's Chinese Theater, and frequent references to Hedy Lamarr. The cartoon The Flintstones depicts many modern appliances in a prehistoric setting.
Even with careful research, Science fiction writers risk anachronism as their works age, because of things they failed to predict: many books nominally set in the mid-21st century assume the continuing existence of the Soviet Union, for example.
With the detail required for a modern historical movie it is easy to introduce anachronisms. The 1995 hit film Apollo 13 contains numerous errors including the use of the incorrect NASA logo and the appearance of The Beatles' "Let It Be" album a month before it was actually released.