Sorry, that was not explained exactly: I didn't mean the derivation of the two words was similar, just that "normal" also means, mathematically speaking "at right angles to" which is close to the word "normal"'s derivation.. perhaps "orthogonal" would have been the better word to use.Actually the Latin meant "plumb" (see the Oxford Latin Dictionary). And as for etymology:
"Middle French, French perpendiculaire (adjective) vertical (1380 as perpendiculer ; also in Middle French as perpendiculier ), at a right angle (1637), (noun) a straight line at right angles to a line or plane (1567) and its etymon classical Latin perpendiculāris situated at right angles to a line or plane, a straight line at right angles to a line or plane < perpendiculum perpendicle n. + -āris -ar suffix1. Compare Old Occitan perpendicular (c1350), Catalan perpendicular (1512), Spanish perpendicular (a1428 or earlier), Italian perpendicolare (14th cent. as perpendiculare)."
(OED)
Mathematicians who don't use perpendicular tend to use orthogonal (which means "right/correct angle"), or refer to a "norm", not "normal".
Right meaning, wrong derivation. Perpendicular isn't etymologically related to "norma" but is closer to the meaning of perpendicular, hence the etymology.
Very true (and quite important).