A baby’s inability to understand that objects gone from sight still exist is commonly referred to as:

The concept referred to in the question is known as object permanence. This cognitive development milestone occurs typically between 4 to 7 months of age when infants start to realize that objects continue to exist even when they are no longer within their field of vision. Understanding object permanence is crucial for an infant's cognitive growth as it lays the foundation for later skills in memory and problem-solving.

When an infant has not yet developed object permanence, they may seem perplexed when an object is removed from their view, failing to search for it. This ability to recognize that an object is still present, despite being out of sight, is fundamental to the child's understanding of the world around them. It aids in the development of trust and security, as children learn that their caregivers and familiar things remain in their lives even when not immediately observable.

Developing object permanence also influences a child's social interactions, as they begin to understand that others can help fulfill their needs even when they are not visible at the moment. This is a significant developmental milestone in an infant’s early life and highlights the importance of caregivers in supporting and engaging with infants to promote this understanding.