-ency

see -ance.

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].