Sincerity is the quality of being sincere: you really mean the words you say, and your face and your actions match them. If you thank someone with sincerity, it is not an empty polite line; you are glad. We talk about the sincerity of an apology, a promise, or a feeling. It is a slightly formal noun. In warm everyday talk people more often say someone is being sincere, or that they really mean it. Though actually, sincerity is about honesty of feeling, not about being right. You can be sincere and still be wrong. People also argue about “performed” sincerity — a speech that sounds from the heart because it was practised. The word itself still points to the real thing: no gap between the inside and the words.