Strong's - "Feminine of the same as G647; defection from truth (properly the state), (“apostasy”): - falling away, forsake."
Thayer's - "1) a falling away, defection, apostasy"
Liddell/Scott - "ἀπο-στᾰσία, ἡ, late form for ἀπόστασις, defection, revolt, v.l. in D.H.7.1, J.Vit.10, Plu.Galb.1...