This is the term that most private medical insurance policies use for hospital treatment that requires a stay of one or more nights.
If you have a basic policy you normally should be insured for staying in as an in-patient either at a private hospital, or the private wing of an NHS hospital depending on your type hospital list. Your health insurance policy will usually also cover any treatment or investigations related to your stay in hospital. The whole point of the insurance is to get you back to full health as quickly as possible, and therefore all aspects of your treatment are usually covered.