How about "em"?

Don't get em started!

Em is the vertical measurement, top to bottom, while the quad is the same measurement squared.

... or em square.

We also used to call it an M-quad, as a space, equal to the square of the point size in foundry type--a quad in 12 point metal was one pica by one pica by space height high. In the digital world, all this goes away because nobody agrees on things any more. It used to be simple, it was a block of lead and that was the end of it :-)

>nobody agrees on things any more

I don't agree with that :)

The measurement from the top to the bottom of the characters in a font is the "point size". There is no simple term in English for the space from ascender or cap height, whichever is greater, to the descender depth, which would exclude possible small amounts of white space in the font in order to permit alignment of the baseline.

If you mean a horizontal distance equivalent to the vertical point size, that is called an "em". In fact, instead of measuring the width of a column of type in picas, the same unit of horizontal distance is called a "pica em" to make it clear that a horizontal distance is discussed.

Thanks all for your remarks. As a see I haven’t been totaly clear in my initial question. I did mean neither the vertical measure (= point size) nor the horizontal one (“em”), but the square unit which results from both. So speaking, “em square” or “M quad” seems approriate to me.

Traditionally, the term "quad" refers to physical chunks of lead used as spacing. The term "em square" now refers to the square on which digital type is drawn, and determines the vertical spacing, before leading.