What term describes the use of letters to represent each speech sound in a language?

The term that best describes the use of letters to represent each speech sound in a language is "phonetic." This term refers to the relationship between the sounds of spoken language and the written symbols used to convey those sounds. In phonetics, each letter or combination of letters corresponds to specific sounds, allowing for accurate pronunciation and understanding of words. This is foundational in linguistics and language instruction, as it helps learners understand how to decode words based on their sounds, facilitating reading and communication. Phonics, while related, specifically refers to a method of teaching reading that focuses on the relationships between letters and sounds rather than the concept of representing speech sounds through letters as a whole.