A phonogram is a visual symbol that represents a sound:
phono, the Greek word for "sound," +
gram, from Greek for a letter or something written
The 26 letters of the alphabet are phonograms. However, they are not the only ones. Forty-nine multi-letter phonograms like igh, ch, and th are also used regularly to represent sounds in English words.
Together, these 75 phonograms are the basic building blocks of written English.
Learn more: What Are Phonograms?