According to the Jargon File

Jargon File

The Jargon File is a glossary of computer programmer slang. The original Jargon File was a collection of terms from technical cultures such as the MIT AI Lab, the Stanford AI Lab and others of the old ARPANET AI/LISP/PDP-10 communities, including Bolt, Beranek and Newman, Carnegie Mellon...

, Smoking clover is a computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 display hack

Display hack

A display hack is a computer program with similar purpose to a kaleidoscope: to make pretty pictures . Famous display hacks include munching squares and smoking clover. Some display hacks can be also implemented by creating text files which contain numerous escape sequences for a text terminal to...

, originally created by Bill Gosper

Bill Gosper

Ralph William Gosper, Jr. , known as Bill Gosper, is an American mathematician and programmer from Pennsauken Township, New Jersey...

. Several converging lines are drawn on a color monitor

Computer display

A monitor or display is an electronic visual display for computers. The monitor comprises the display device, circuitry, and an enclosure...

 in such a way that every pixel

Pixel

In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....

 struck has its color incremented—altered to the next hue

Hue

Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...

 up or down. The color map is then repeatedly rotated.

The result of this is a rainbow

Rainbow

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines on to droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc...

-hued, shimmering four-leaf clover

Four-leaf clover

The four-leaf clover is an uncommon variation of the common, three-leaved clover. According to tradition, such leaves bring good luck to their finders, especially if found accidentally...

. The program has been described as "psychedelic

Psychedelic

The term psychedelic is derived from the Greek words ψυχή and δηλοῦν , translating to "soul-manifesting". A psychedelic experience is characterized by the striking perception of aspects of one's mind previously unknown, or by the creative exuberance of the mind liberated from its ostensibly...

", and Gosper joked about keeping it a secret from the Food and Drug Administration

Food and Drug Administration

The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

 (FDA) due to its "hallucinogenic

Psychedelics, dissociatives and deliriants

This general group of pharmacological agents can be divided into three broad categories: psychedelics, dissociatives, and deliriants. These classes of psychoactive drugs have in common that they can cause subjective changes in perception, thought, emotion and consciousness...

properties".

Source code for Linux to reproduce the effect is available here - the resulting program is best run with the -r flag.