Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 adj. Forming a relationship with other parts or quantities; being in proportion.
 adj. Properly related in size, degree, or other measurable characteristics; corresponding: Punishment ought to be proportional to the crime.
 adj. Mathematics Having the same or a constant ratio.
 n. One of the quantities in a mathematical proportion.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adj. At a constant ratio (to). Two magnitudes (numbers) are said to be proportional if the second varies in a direct relation arithmetically to the first.
 adj. In proportion (to), proportionate. More common in US than UK.
from the GNU version of the Collaborative International Dictionary of English
 adj. Having a due proportion, or comparative relation; being in suitable proportion or degree.
 adj. Relating to, or securing, proportion.
 adj. Constituting a proportion; having the same, or a constant, ratio.
 adj. instruments used in making copies of drawings, or drawings of objects, on an enlarged or reduced scale.
 n. See under Proportional, etc.
 n. Any number or quantity in a proportion.
 n. The combining weight or equivalent of an element.
from The Century Dictionary and Cyclopedia
 Based upon proportion; pertaining to or having proportion.
 According to or having a due proportion; being in suitable proportion or degree.
 In mathematics, having the same or a constant ratio: as, proportional quantities.
 A scale for preserving the proportions of drawings or parts when changing their size.
 n. A quantity in proportion.
 n. A table of proportional parts.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. one of the quantities in a mathematical proportion
 adj. having a constant ratio
 adj. properly related in size or degree or other measurable characteristics; usually followed by `to'
Etymologies
Sorry, no etymologies found.
Examples

T, sea level rise would also contain a term proportional to the time derivative of temperature, dT/dt.

IRV is a bandaid, trying to fix fatal flaws in proportional representation.

I know that the Liberal Democrats want STV, but now, I think that as the Make My Vote Count campaign rightly recognises, any change to the system that makes it slightly more proportional is worth voting for.
Are we going to miss the chance for proper reform?… « My Liberal Democrat Political Ramblings…

But what I really recommend is to watch the video that first got me interested in proportional representation.
hughstimson.org » Blog Archive » John Cleese’s VHSEra Pitch for Proportional Representation

In fact, states that want to devolve some power to counties and municipalities are nonetheless constitutionally required to maintain proportional representation at the state level.
Matthew Yglesias » Malapportionment Is Destroying the Planet

That style uses the traditional three periods (or, in proportional typefaces, one ellipsis mark) when a quotation is missing a phrase from within a sentence.

Now, with laser and inkjet printers setting type in proportional fonts, we can go back to using em dashes as they were meant to be used  but people are still typing double hyphens.

Bringing in proportional representation does not represent a complete programme for a fiveyear parliament.

It includes not only fourterm proportional relationships of the type A: B:: C: D (for which the Greek term is ἀναλογία), but also both explicit and implicit comparisons, for example the use of models (παραδείγματα) and of images (εἰκόνεσ).

To differentiate them, STV used for multiwinner elections is sometimes called proportional representation through the single transferable vote, or PRSTV.
jmjarmstrong commented on the word proportional
JM has observed that the hardness of the butter is directly proportional to the softness of the bread.
July 17, 2011