At the time the Quran was revealed, the Arabic letters that we know of today was different. As I understand it, they did not have the concept of letter in the meaning we understand today.
For example, the letters in the second row are, from left to right,
ba, ta, tha. Originally, the symbols did not have those dots, and in written language the symbol in the first row was used for all three of them. Whether it was the first or the second or the third was understood from the context. (Dots were added centuries later so that people could recognize/understand easily.) Here is an image of a page of the Quran with the old Arabic letters:
Here is the definition of letter in The Concise Oxford Dictionary of Linguistics (2nd Ed.)
letter Originally of a unit of speech having both a written form and a phonetic value. Thus, in ancient accounts, the letter (Latin litera) was the smallest unit into which utterances, defined as movements of air that are representable in writing, were analysed. Now, as in everyday usage, of a character in writing only, especially in an alphabetic system.
My question: Is there a term for signs/symbols that represent more than one letter?