An intensive care unit (ICU), also called a critical care unit (CCU) or intensive therapy unit (ITU), is a special hospital department that provides intensive care medicine to patients with severe or life-threatening illnesses and injuries.
ICU patients need constant monitoring and organ support, such as mechanical ventilation, and are cared for by specially trained doctors and nurses with a high staff-to-patient ratio. Many hospitals also run specialized units, such as neonatal (NICU), pediatric, cardiac, and neurological ICUs.
Source: Intensive care unit, Wikipedia