Rarefaction (noun)

1. The act or process of making something rare or less dense.
2. The process by which a fluid or gas becomes less dense as a result of pressure reduction.

Origin:

Late 17th century: from rare + -faction, on the pattern of condensation.

Examples:

1. The rarefaction of the gas caused it to expand and cool.
2. The rarefaction of the gas was an important factor in the formation of stars.
3. The rarefaction of the gas in the balloon caused it to rise in the air.
4. Sound travels faster in rarefaction, because the molecules are more widely spaced.
5. The rarefaction of the air caused a reduction in air pressure and an increase in temperature.