Sincere is an adjective meaning free from pretense or deceit; proceeding from genuine feelings.
Common usages include:
1. Sincere apology - a heartfelt expression of regret
2. Sincere thanks - genuine gratitude
3. Sincere person - someone who is honest and straightforward
Synonyms: genuine, heartfelt, earnest, honest, truthful
Antonyms: insincere, fake, dishonest, deceitful