Baseline

It is the reference everything else in a font is measured from: the x-height above it, the cap height above that, the descender below. When two different fonts are set on one line — a heading and an icon, a word in another script, a monospace figure inside a sentence — it is the baseline that lines them up, and a mismatch is what makes an inline icon look like it is falling off the line.

Round letters deliberately break it. An o drawn to sit exactly on the baseline looks smaller than a flat-bottomed n beside it, so it is drawn slightly deeper — an overshoot of one or two per cent. The same goes for pointed letters like v and w. It is a correction for how the eye works rather than for how the geometry works, and every properly drawn face has it.