Definitions
from The American Heritage® Dictionary of the English Language, 5th Edition.
 adjective Forming a relationship with other parts or quantities; being in proportion.
 adjective Properly related in size, degree, or other measurable characteristics; corresponding.
 adjective Mathematics Having the same or a constant ratio.
 noun One of the quantities in a mathematical proportion.
from The Century Dictionary.
 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.
 noun A quantity in proportion.
 noun A table of proportional parts.
from the GNU version of the Collaborative International Dictionary of English.
 noun (Math.) Any number or quantity in a proportion.
 noun (Chem.), obsolete The combining weight or equivalent of an element.
 adjective Having a due proportion, or comparative relation; being in suitable proportion or degree.
 adjective Relating to, or securing, proportion.
 adjective (Math.) Constituting a proportion; having the same, or a constant, ratio.
 adjective logistic logarithms. See under
Logistic .  adjective a scale on which are marked parts proportional to the logarithms of the natural numbers; a logarithmic scale.
 adjective (Draughting) instruments used in making copies of drawings, or drawings of objects, on an enlarged or reduced scale.
from Wiktionary, Creative Commons Attribution/ShareAlike License.
 adjective At a constant ratio (to). Two
magnitudes (numbers) are said to be proportional if the second varies in a direct relationarithmetically to the first.  adjective In proportion (to),
proportionate . More common in US than UK.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 noun one of the quantities in a mathematical proportion
 adjective having a constant ratio
 adjective 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.

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

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

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

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

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

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.
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