The letter "d" is the fourth letter in the English alphabet and has several synonyms, including "dee," "delta," and "datum." "Dee" is a short form of the letter and often used in casual conversations. "Delta" is used in the naming of bodies of water that form a triangular shape, such as the Nile Delta. "Datum" refers to a single piece of information, often used in the context of scientific or mathematical data. Additionally, "d" can also be represented by the phonetic sounds of "duh" or "da." Overall, the synonyms for the letter "d" show the versatility and flexibility of language in expressing ideas and concepts.