A crop circle, crop formation, or corn circle is a pattern created by flattening a crop, usually a cereal. The term was first coined in the early 1980s. Crop circles have been described as all falling "within the range of the sort of thing done in hoaxes" by Taner Edis, professor of physics at Truman State University.

Although obscure natural causes or alien origins of crop circles are suggested by fringe theorists, there is no scientific evidence for such explanations, and all crop circles are consistent with human causation. In 1991, two hoaxers, Doug Bower and Dave Chorley, took credit for having created over 200 crop circles throughout England, in widely-reported interviews. The number of reports of crop circles increased substantially after interviews with them. In the United Kingdom, reported circles are not distributed randomly across the landscape, but appear near roads, areas of medium to dense population, and cultural heritage monuments, such as Stonehenge or Avebury. They usually appear overnight. Nearly half of all crop circles found in the UK in 2003 were located within a 15 km (9.3 mi) radius of the Avebury stone circles.

In contrast to crop circles or crop formations, archaeological remains can cause cropmarks in the fields in the shapes of circles and squares, but these do not appear overnight, and are always in the same places every year.

A 1678 news pamphlet The Mowing-Devil: or, Strange News Out of Hartfordshire describes a crop whose stalks were cut rather than bent. (see folklore section).

In 1686, an English naturalist, Robert Plot, reported on rings or arcs of mushrooms (see fairy rings) in The Natural History of Stafford-Shire, proposing air flows from the sky as a cause. In 1991, meteorologist Terence Meaden linked this report with modern crop circles, a claim that has been compared with those made by Erich von Däniken.

An 1880 letter to the editor of Nature by amateur scientist John Rand Capron describes how several circles of flattened crops in a field were formed under suspicious circumstances and possibly caused by "cyclonic wind action", stating "as viewed from a distance, circular spots (...) they all presented much the same character, viz, a few standing stalks as a centre, some prostrate stalks with their heads arranged pretty evenly in a direction forming a circle round the centre, and outside there a circular wall of stalks which had not suffered".

In 1932, archaeologist E. C. Curwen observed four dark rings in a field at Stoughton Down near Chichester, but could examine only one: "a circle in which the barley was 'lodged' or beaten down, while the interior area was very slightly mounded up."

In Fortean Times, David Wood reported that in 1940 he made crop circles near Gloucestershire using ropes.