Sound Transmission Class (or STC) is an integer

Integer

The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

 rating of how well a building

Building

In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...

 partition

Wall

A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...

 attenuates airborne sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

. In the USA, it is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90). Outside the USA, the Sound Reduction Index (SRI)

Sound Reduction Index

The sound reduction index is used to measure the level of sound insulation provided by a structure such as a wall, window, door, or ventilator. It is defined in the series of international standards ISO 140 , or the regional or national variants on these standards. In the United States, the sound...

 ISO standard is used.

The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide.
The ASTM test methods have changed every few years.