The knowledge or awareness that objects continue to exist when they are out of sight, either through being in view but out of the visual field, or through being hidden by another object. According to Jean Piaget, infants develop the beginnings of object permanence at 9 months, with further progress taking place up to 24 months. More recent, but contentious, accounts suggest that something like object permanence is present by 3 or 4 months or that it is an innate ability.
See A-not-B error/task, Bayley Scales of Infant and Toddler Development, Embodiment, Innate 1, Innate 2, Mental image, Numerical identity, Object identity, Search errors in infancy