noun
grapheme
Linguistics term for the smallest functional unit of a writing system, such as a letter, character, or diacritic.
日本語の書記素には、漢字や仮名が含まれる。
Japanese graphemes include kanji and kana.
アルファベットの各文字は一つの書記素と見なされる。
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.