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also rar·i·fy (râr′ə-fī′)
v. rar·e·fied, rar·e·fy·ing, rar·e·fies also rar·i·fied or rar·i·fy·ing or rar·i·fies
1. To make thin or less dense, as air.
2. To purify or refine: "The [medieval] Italians ... rarefy the image of the lady, treating her as a quasi-celestial figure" (Marcia L. Colish).
To become thin or less dense.

[Middle English rarefien, from Old French rarefier, from Medieval Latin rārificāre, alteration of Latin rārēfacere : rārus, rare + facere, to make; see dhē- in Indo-European roots.]

rar′e·fi′a·ble adj.