G5516 χξϛ (chx stigma)

KJV renderings: six hundred threescore and six
Strong's definition: 666 as a numeral
Derivation: the 22nd, 14th and an obsolete letter (G4742 (στίγμα) as a cross) of the Greek alphabet (intermediate between the 5th and 6th), used as numbers; denoting respectively 600, 60 and 6;

From James Strong's public-domain dictionaries (G), JSON edition by OpenScriptures (CC-BY-SA). Every dotted-underline word in the text links here.