thank-you letter; letter of appreciation; certificate of appreciation
A formal written expression of gratitude, often presented as a certificate or official letter.
A letter of appreciation was presented by the mayor for the volunteer work.
感謝状 is a certificate expressing thanks or appreciation, whereas 表彰状 formally recognizes merit or accomplishment.
Compound of 感謝 (kansha, 'gratitude') and 状 (jō, 'letter' or 'certificate').