grapheme
Linguistics term for the smallest functional unit of a writing system, such as a letter, character, or diacritic.
Japanese graphemes include kanji and kana.
Each letter of the alphabet is considered a grapheme.
文字 is a broader, everyday term for a letter or character, while 書記素 is a technical linguistics term.
Compound of 書記 (writing) and 素 (element), likely a calque from Western linguistic terminology.