rarefaction n.

"act or process of making rare or expanding a body of matter (originally chiefly gases) so that a smaller number of particles occupy the same space," c. 1600, from French raréfaction or directly from Medieval Latin rarefactionem (nominative rarefactio), noun of action from past-participle stem of Latin rarefacere "make rare or thin" (see rarefy). Rarefication (1610s) also is used. Rarefactive is attested from early 15c.