When a baby dies in a crib during sleep it's called SIDS.
That is sudden infant death syndrome and the cause of the death of the baby in the crib is unknown and unexplained.
SIDS is also known as crib death as well since the baby often dies of SIDS in the crib however sometimes a baby may suddenly die of SIDS while in the parents arms or anywhere else they may be sleeping.
SIDS occurs in babies of less than 1 year of age.