Measure

It is counted in characters because that is what the eye deals with. A line that is too long makes the return sweep unreliable and readers start losing their place or re-reading the line they just finished; a line that is too short breaks the rhythm and forces a return every few words. Neither failure announces itself — the reader simply finds the page tiring and does not know why.

The number is not a rule so much as a range, and it moves with the leading: a long measure survives if the lines are opened up, a short one tolerates tighter setting. On the web the practical way to set it is a max-width in ch units on the text column, so the measure holds as the font size changes rather than drifting with the window.