word-forming element denoting quality or state, from Latin -entia. Derivatively identical with -ence; also see -ancy. The slight difference of sense is that -ence can and -ency cannot mean "an act of ____."
word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past-participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance). But English also has kept many pairs of words in -ance and -ancy (radiance/radiancy, etc.). Though typically one of the two forms has been more common, both were kept "as metrically useful or rhetorically effective" [Fowler, 1926].
<a href="https://www.etymonline.com/word/-ency">Etymology of -ency by etymonline</a>
Harper, D. (n.d.). Etymology of -ency. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/-ency