Strong's Concordance

Original word: ἀφελότης
Transliteration: aphelotés
Definition (short): sincerity
Definition (full): simplicity

NAS Exhaustive Concordance

Word Origin: from aphelés (without a stone, even, smooth)
Definition: simplicity
NASB Translation: sincerity (1).
NAS Exhaustive Concordance of the Bible with Hebrew-Aramaic and Greek Dictionaries.
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Strong's Exhaustive Concordance

From a compound of a (as a negative particle) and phellos (in the sense of a stone as stubbing the foot); smoothness, i.e. (figuratively) simplicity -- singleness.

see GREEK a

KJV: And they, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart,
NASB: Day by day continuing with one mind in the temple, and breaking bread from house to house, they were taking their meals together with gladness and sincerity of heart,