The Collaborative International Dictionary of English v.0.48:

Proportion \Pro*por"tion\, n. [F., fr. L. proportio; pro before + portio part or share. See Portion.] [1913 Webster] 1. The relation or adaptation of one portion to another, or to the whole, as respect magnitude, quantity, or degree; comparative relation; ratio; as, the proportion of the parts of a building, or of the body. [1913 Webster] The image of Christ, made after his own proportion. --Ridley. [1913 Webster] Formed in the best proportions of her sex. --Sir W. Scott. [1913 Webster] Documents are authentic and facts are true precisely in proportion to the support which they afford to his theory. --Macaulay. [1913 Webster] 2. Harmonic relation between parts, or between different things of the same kind; symmetrical arrangement or adjustment; symmetry; as, to be out of proportion. "Let us prophesy according to the proportion of faith." --Rom. xii. 6. [1913 Webster] 3. The portion one receives when a whole is distributed by a rule or principle; equal or proper share; lot. [1913 Webster] Let the women . . . do the same things in their proportions and capacities. --Jer. Taylor. [1913 Webster] 4. A part considered comparatively; a share. [1913 Webster] 5. (Math.) (a) The equality or similarity of ratios, especially of geometrical ratios; or a relation among quantities such that the quotient of the first divided by the second is equal to that of the third divided by the fourth; -- called also geometrical proportion, in distinction from arithmetical proportion, or that in which the difference of the first and second is equal to the difference of the third and fourth. [1913 Webster] Note: Proportion in the mathematical sense differs from ratio. Ratio is the relation of two quantities of the same kind, as the ratio of 5 to 10, or the ratio of 8 to 16. Proportion is the sameness or likeness of two such relations. Thus, 5 to 10 as 8 to 16; that is, 5 bears the same relation to 10 as 8 does to 16. Hence, such numbers are said to be in proportion. Proportion is expressed by symbols thus: [1913 Webster] a:b::c:d, or a:b = c:d, or a/b = c/d. [1913 Webster] (b) The rule of three, in arithmetic, in which the three given terms, together with the one sought, are proportional. [1913 Webster] Continued proportion, Inverse proportion, etc. See under Continued, Inverse, etc. Harmonical proportion or Musical proportion, a relation of three or four quantities, such that the first is to the last as the difference between the first two is to the difference between the last two; thus, 2, 3, 6, are in harmonical proportion; for 2 is to 6 as 1 to 3. Thus, 24, 16, 12, 9, are harmonical, for 24:9::8:3. In proportion, according as; to the degree that. "In proportion as they are metaphysically true, they are morally and politically false." --Burke. [1913 Webster]