Strong's numbers
ἀποστασία (G646)
apostasía
ap-os-tas-ee'-ah
feminine of the same as ἀποστάσιον; defection from truth (properly, the state) ("apostasy"):--falling away, forsake.
Abbott–Smith
† ἀποστασία, -ας, ἡ (< ἀφίστημι), [in LXX for מַעַל, etc.;]
defection, apostasy, revolt; in late Gk. (MM, Exp., viii; Lft., Notes, 111; Cremer, 308) for cl. ἀπόστασις, freq. in sense of political revolt, in LXX (e.g. Jos 22:22, II Ch 29:19, Je 2:19) and NT always of religious apostasy: Ac 21:21, II Th 2:3.†
From G. Abbott-Smith, A Manual Greek Lexicon of the New Testament (1922), in the public domain.
2 occurrences
Shown in the King James Version (KJV), whichever translation you read in — it is the text these numbers are tagged against.
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Acts 21:21
And they are informed of thee, that thou teachest all the Jews which are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children, neither to walk after the customs.
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2 Thessalonians 2:3
Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;