cot death 
noun British
The unexplained death of a baby in its sleep: cot deaths in England and Wales have dropped for the third year in succession [mass noun]: the risk of cot death could be greatly reduced...
- A thorough investigation into the cause of death is carried out after a cot death.
- Now re-housed, the couple said the cot death of their baby had changed their lives, but had also brought them closer together.
- There it is estimated that the chances of a randomly chosen baby dying a cot death are 1 in 1,303.