Glyph
A glyph is a carved figure or character, incised or in relief; a carved pictograph; hence, a pictograph representing a form originally adopted for sculpture, whether carved or painted. Augustan English scholars of the early 18th century, imitating French antiquaries, adopted glyph from the Greek word meaning a "carving." Compare the carved and incised "sacred glyphs" hieroglyphs, which have had a longer history in English dating from the first Elizabethan translation of Plutarch (who adapted "hieroglyphic" as a Latin adjective). But "glyph" first came to widespread European attention with the engravings and lithographs from Frederick Catherwood's drawings of undeciphered glyphs of the Maya civilization in the early 1840s. "Glyphs" still bring connotations of Maya glyphs to mind.
In Typography, a glyph is an allograph: a particular graphical representation of a Grapheme, or sometimes several graphemes in combination, or only a part of a grapheme. In Computing as well as typography, the term character refers to a grapheme or grapheme-like unit of text, as found in Natural language writing systems (scripts). A character or grapheme is a unit of Text, whereas a glyph is a graphical unit.
For example, the sequence ffi contains three characters, but will be represented by one glyph in TeX, since the three characters will be combined into a single ligature. Conversely, some typewriters require the use of multiple glyphs to depict a single character (for example, two hyphens in place of a Dash, or an overstruck Apostrophe and period in place of an Exclamation mark).
Most glyphs in typography originate from the carved and cast characters of a Typeface, also called a font. In computing, font refers to a typeface manifesting as an indexed collection of glyphs or glyph-rendering instructions, and associated information that facilitates rendering mapping characters to glyphs and for rendering glyphs in different sizes. For a given typeface or font, each character typically corresponds to a single glyph. However, this is not always the case, especially in a font used for a language with a large alphabet or complex writing system, where one character may correspond to several glyphs, or several characters to one glyph.
In Graphonomics, the term glyph is used for a non-character, i.e., either a sub-character or multi-character pattern.