A notelet is a small folded piece of paper, often with a picture on the front, for writing a short letter or a thank-you. It is mainly a British word; American speakers are more likely to say note card. It is smaller and lighter than a full letter on notepaper, and more personal than an email. Shops still sell them, though the word can sound a little old-fashioned. The idea is a tiny letter, not a tiny notebook.