Retention of vision is a conjuring term referring to the perceived image

Visual perception

Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

 of an object during a vanish (usually associated with vanishing coins). In a retention of vision vanish

Retention of vision vanish

In the retention of vision vanish, the magician places a coin or small object between the fingers and the thumb of the right hand. The left hand is palm up with the palm slightly tilted towards the audience. The right approaches and displays the object on the left palm for a second or so, held on...

, the spectator sees a coin placed on a palm and as the fingers are closed around the coin it is secretly removed, but because the retina still retains a fleeting image of the coin for a fraction of a second, the spectator will be convinced that the coin was placed in the hand, when in fact it wasn't.