Glyph

Three words get used for the same thing and they are not the same thing. A character is the unit of writing: the letter a, whatever it looks like. A codepoint is the number a character is stored as, U+0061 for that a. A glyph is a shape a typeface draws, and it exists in the font file rather than in the text. Set a sentence in two faces and the characters are identical while every glyph has been replaced, which is the whole reason a font swap changes the length of a line.

The mapping between the two is many to many in both directions. One character can have several glyphs in a single face: a lining figure and an oldstyle one, a small cap and a full capital, a swash alternate for the last letter of a word, a contextual form that only appears after certain letters. And several characters can share one glyph, which is what a ligature is: the pair f and i stays two characters in the string, two codepoints in the file, and one drawing on the screen. Nothing about the text changed. The font simply had a better answer for that sequence, and OpenType features are the switches that let it give one.

The distinction bites wherever software has to count. String length counts codepoints, not shapes, so a fused fi is two and a family emoji built from several codepoints joined together is five or six while being unmistakably one picture. The caret has to move by shapes rather than by codepoints, or an arrow key lands somewhere with no visible position, which is why text engines work in clusters instead. Scripts like Devanagari and Arabic make the gap obvious rather than occasional, since a written syllable there is routinely one glyph assembled from several characters, and any tool that assumed one shape per character breaks the first time it meets one.

For anyone shipping fonts, glyphs are also the unit of cost. Subsetting a web font means keeping the glyphs a site actually needs and discarding the rest, which is where the real weight goes, and a face advertised as having thousands of glyphs is advertising its coverage of scripts and alternates rather than anything about its letters. It is worth checking what a file really contains, because a feature the font has no glyphs for cannot happen: the tag is accepted, nothing is substituted, and the page looks exactly as it did before.