A glyph (pron.: /ˈɡlɪf/) is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written.
In archæology, a glyph is a carved or inscribed symbol. It may be a pictogram or ideogram, or part of a writing system such as a syllable, or a logogram.
A glyph is "the specific shape, design, or representation of a character". It is a particular graphical representation, in a particular typeface, of an element of written language, which could be a grapheme, or part of a grapheme, or sometimes several graphemes in combination (a composed glyph). If there is more than one allograph of a unit of writing, and the choice between them depends on context or on the preference of the author, they now have to be treated as separate glyphs, because mechanical arrangements have to be available to differentiate between them and to print whichever of them is required.
More information on glyphs in general on the Wikipedia page [1]. The Wikimedia category Glyphs is [2]. There are a number of other websites.
See also Frederick Catherwood and Arborglyph.