The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At , the speed of sound in air is about , or one km in  or one mile in . It depends strongly on temperature as well as the medium through which a sound wave is propagating. At , the speed of sound in air is about . More simply, the speed of sound is how fast vibrations travel.
The speed of sound in an ideal gas depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior.
In colloquial speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: typically, sound travels most slowly in gases, faster in liquids, and fastest in solids. For example, while sound travels at  in air, it travels at  in water (almost 4.3 times as fast) and at  in iron (almost 15 times as fast). In an exceptionally stiff material such as diamond, sound travels at , about 35 times its speed in air and about the fastest it can travel under normal conditions.
In theory, the speed of sound is actually the speed of vibrations.
Sound waves in solids are composed of compression waves (just as in gases and liquids) and a different type of sound wave called a shear wave, which occurs only in solids. Shear waves in solids usually travel at different speeds than compression waves, as exhibited in seismology. The speed of compression waves in solids is determined by the mediums compressibility, shear modulus, and density. The speed of shear waves is determined only by the solid materials shear modulus and density.
In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure for the speed of an object moving through the medium. The ratio of the speed of an object to the speed of sound (in the same medium) is called the objects Mach number. Objects moving at speeds greater than the speed of sound (') are said to be traveling at supersonic speeds.