Sound insulation is the term used when talking about how much sound passes from one space to another. There are two types. Airborne sound insulation relates to noise such as speech and music, and is relevant when designing both walls and floors. Impact sound insulation is considered in the design of floors and ceilings, and relates to the control of noise from (typically) people walking on the floor above being heard in the space below.
This is sometimes referred to as ‘soundproofing’, but it is important to understand that nothing is ever truly soundproof! The key is to control noise transfer to within an acceptable level, based on the typical activities which will occur in the spaces under consideration.