In architecture, a folly is an extravagant, frivolous or fanciful building, designed more for artistic expression than for practicality.
Ferdinand Cheval.
However, very few follies are completely without a practical purpose. Apart from their decorative aspect, many originally had a use which was lost later, such as hunting towers. Headley & Meulenkamp in Follies Grottoes & Garden Buildings defined a folly as a "misunderstood building".
Follies are often found in parks or large grounds of houses and stately homes. Some were deliberately built to look partially ruined. They were especially popular from the end of the 16th century to the 18th century.