字位或字素（英语：grapheme）这个术语，是由语音学里的“音位”（音素）类推到文本学的。在拼音文本系统当中，“字位”是最小的,数量最少的区别性单位。“字位”是一个抽象单位。例如：字位<a>,无论它的形式是A，a，或是α，它们都区别于b,c,d等等，所以，A，a，α仍然属于一个单位，为组成<a>字位的成员。组成一个字位的成员叫“字位变体”（字素变体）（allographs）.

计算一个文本系统的规模时，是以字位这个最小单位来计算的，而不是以字位变体。如：英文本母表当中虽然印有52个大小写变体，但是仍然说英文本母有26个，即26个字位。

A grapheme is the smallest unit used in describing the writing system of any given language, originally coined by analogy with the phoneme of spoken languages. A grapheme may or may not carry meaning by itself, and may or may not correspond to a single phoneme. Graphemes include alphabetic letters, typographic ligatures, Chinese characters, numerical digits, punctuation marks, and other individual symbols of any of the world's writing systems.