Zener cards refer to a deck of 25 cards bearing various symbols and used in early parapsychological research.
Zener cards are a set of cards used in parapsychology experiments to test for extrasensory perception (ESP) or clairvoyance. The cards were created by psychologist Karl Zener in the 1930s and consist of five different symbols: a circle, square, star, cross, and wavy lines.
In experiments, a participant is asked to guess which symbol is on the card without looking at it, and the results are recorded to see if the guesses are statistically significant above chance levels. The use of Zener cards in parapsychology research has been controversial, with some studies suggesting evidence for ESP and others finding no support for the phenomenon.