means The size, amount, or degree of one thing measured in relation to another, or the balanced relationship between parts of a whole.

from From Latin proportio, built from pro- ('for, according to') and portio ('a share, a portion'). So at root it's 'according to each one's share' — the idea that things relate by their measured slice. Roman writers used it to translate the Greek mathematicians' notion of analogia, ratio between quantities, and it slipped into English through Old French in the late Middle Ages, carrying both the cold arithmetic of ratios and the warmer sense of harmony and fit.