A formal, somewhat literary noun for wholehearted sincerity or genuine devotion. Often used in set phrases like 誠心誠意.
He tackled the problem with utmost sincerity.
I offer my sincere apologies.
loyalty; faithfulness
in all sincerity; with one's whole heart; whole-hearted devotion
Compound of 誠 (true, sincere) and 心 (heart, mind). The exact historical derivation is uncertain, but the word has been used in formal Japanese to express wholehearted sincerity.