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.