Source : Webster's Revised Unabridged Dictionary (1913)

Irreligion \Ir`re*li"gion\, n. [L. irreligio: cf. F.
   irr['e]ligion. See {In-} not, and {Religion}.]
   The state of being irreligious; want of religion; impiety.

Source : WordNet®

irreligion
     n : the quality of not being devout [syn: {irreligiousness}]