衷心 (zhōngxīn) means 'heartfelt, wholehearted' and is used to express sincere wishes, congratulations, or respect, often in formal or written contexts. It is similar to 诚挚 (chéngzhì 'sincere, cordial') but 衷心 emphasizes the depth of feeling from the heart, while 诚挚 focuses on an honest and sincere attitude. 由衷 (yóuzhōng) is also close but is more often used as an adverb (由衷地) meaning 'from the bottom of one's heart'.