The term itself is derived from the Greek words para, meaning something faulty, wrong, instead of, and the noun eidōlon, meaning image, form, or shape. Pareidolia is a type of apophenia, which is a more generalized term for seeing patterns in random data.

Pareidolia often has religious overtones. A prime example of their connection is the Shroud of Turin, a cloth bearing the image of a man — which some claim to be Jesus — who appears to have suffered trauma consistent with crucifixion.

The negative image was first observed in 1898, on the reverse photographic plate of amateur photographer Secondo Pia, who was allowed to photograph it while it was being exhibited in the Turin Cathedral.

Some visitors to St. Mary's in Rathkaele, Ireland, say a tree stump outside of the church bears a silhouette of the Virgin Mary.

On the other side of the world, damage to the Pedra da Gávea, an enormous rock outside Rio de Janeiro, Brazil, created an impression that many interpret as a human face.

In 1977, the appearance of Jesus Christ on a flour tortilla set the international standard for miracle sightings. It happened in the small town of Lake Arthur, New Mexico, 40 minutes south of Roswell.

Diane Duyser of Miami sold a 10-year-old grilled cheese sandwich, which she said bore the image of Jesus, for $28,000 on eBay in 2004 (which equates to about $45,000 in today's money).

There are a number of theories as to the cause of pareidolia. Some experts say it provides a psychological determination for many delusions that involve the senses. They believe pareidolia could be behind numerous sightings of UFOs, Elvis, and the Loch Ness Monster and the hearing of disturbing messages on records when they are played backward.

Carl Sagan, the American cosmologist and author, made the case that pareidolia was a survival tool. In his 1995 book, The Demon-Haunted World – Science as a Candle in the Dark, he argued that this ability to recognize faces from a distance or in poor visibility was an important survival technique. While this instinct enables humans to instantly judge whether an oncoming person is a friend or foe, Sagan noted that it could result in some misinterpretation of random images or patterns of light and shade as being faces.

But we humans have been aware of this phenomenon for a long time. Even Leonardo da Vinci wrote about pareidolia as an artistic device. "If you look at any walls spotted with various stains or with a mixture of different kinds of stones, if you are about to invent some scene you will be able to see in it a resemblance to various different landscapes adorned with mountains, rivers, rocks, trees, plains, wide valleys, and various groups of hills," he explained in a passage in one of his extensive notebooks.