The grapheme appears to be a central concept of grapholinguistics. However, there is no consensus on how it should be defined. Some use the concept of grapheme in their work but fail to give a definition while others altogether reject it. When the concept is defined, it is interpreted either as a written unit which refers to a phoneme (this is termed the referential view), or as a written unit that is lexically distinctive (analogical view), which is tested via written minimal pairs such as and analogously to phonological minimal pairs which can be used to discover phonemes. A problem of these two views is that they are restricted to alphabets. A universal conception of the grapheme inclusive of all types of writing systems would make possible the uniform description and, consequently, the comparison of diverse writing systems. Such a conception is proposed here: Graphemes are units of writing which are (1) lexically distinctive, (2) have linguistic value (mostly by referring to phonemes, syllables, morphemes, etc.), and are (3) minimal. These criteria are characterised in detail, and examples from writing systems such as Arabic, Chinese, Devanāgarī, German, Japanese, Korean, Tamil, and Thai highlight their cross-linguistic applicability.