The smallest unit in written language capable of causing a contrast in meaning. In English they are letters and punctuation marks.

Often A, a and a are all considered the same grapheme, as all are recognized as having the same meaning when put into like contexts. (Cat, CAt, and Cat all have the same meaning). They are allographs.

Other languages (Chinese for example) use logograms. These symbols are both graphemes and morphemes.

The counterpart used in speech is a phoneme.