The symbol that represents a speech sound and is a unit of an alphabet is called a "letter". An alphabet is a set of letters that are used to represent the basic sounds of a language. In English, for example, the alphabet consists of 26 letters: A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, and Z. These letters can be combined in various ways to form words, sentences, and other forms of written language.
The letters of an alphabet are usually arranged in a specific order, such as the traditional Latin alphabet, which is also known as the Roman alphabet. This order is often used as the basis for alphabetizing words in a dictionary or other reference work.
In other languages such as Indian or East Asian Languages, there are other scripts such as Devanagari, Chinese characters etc which are also used as symbols to represent speech sound.
It's worth noting that many writing systems around the world do not use alphabets but symbol system such as hieroglyphics and cuneiform which is not considered as alphabet.
