Font size
How large type is set, measured in pixels across the full height of the letter box.
See it in Typography pairing. Move Heading size and Body size and compare the two.
In plain words
Font size is not the height of one letter. It is the height of an invisible box every letter of the font is drawn in. That box leaves room for tall letters like h and tails like g. Two fonts at the same size can look bigger or smaller, because their letters fill the box differently. On screens, sizes are set in pixels, and reading text is often 16 to 18.