In physics, beauty is a measurement of bonability. A few fine examples would be Audrey Hepburn, sunsets, thin girls, and women with orange skin. It is somewhat analogous to charge, in that the more beauty a woman has, the more money will be charged to a man's credit card. This analogy is not perfect, as detailed below.
The Quantum Theory of Beauty
Beauty, which is usually measured in units of millihelens, is best understood through statistical analysis. Much as, in quantum mechanics, the measurement of a particle's position determines likelihood of the observer finding the particle in a region of space at some future time, the measurement of an object's (woman's) beauty determines the likelihood that the observer will bone the object at some future time.
Beauty is not the only quantity to consider when determining such a probability, however, as other properties may be controlling (see Other (secondary) Factors). In general, a large amount of beauty indicates a low likelihood of boning, a mediocre amount of beauty indicates a very high likelihood of boning, and a small amount of beauty indicates a high likelihood of loneliness, unemployment, masturbation, and homelessness.
According to the Fashion Police, ugliness is the opposite of beauty, and is a fine-able offense in monopoly money.
Beauty and Relativity
The quantum theory of beauty is also a relativistic one, and as such successfully competes with superstring theory in the acquisition of gargantuan research grants from the Federal government. Thus, the well-funded observer must compensate for a variety of factors to correctly determine the beauty of an object (i.e., woman).
Primary Factors affecting Beauty
Alcohol: Alcohol is the most common cause of a distorted beauty measurement. A person under the influence of alcohol will typically measure beauty to be greater than it actually is. Such a person will later make typical statements such as, "OMG, I SLEPT with a homeless woman! Last night!"
Wealth: A wealthy woman may appear more attractive than a poor woman. Women are unaware of this fact. Thus, a rich woman will compensate for her ugliness by buying the observer expensive things. For this reason, it is usually unnecessary to compensate for wealth.
Facial Hair: A truly beautiful woman may sometimes be hidden underneath a furry beard. Shave, Nair™, and reassess.
Mass (or Weight, for you non-physics types): If the object has an anomalously large mass, the object's gravitational field will warp time itself, causing local clocks to slow to a crawl, making painful dinner dates subjectively last far too long.
Other (secondary) Factors
Other factors may augment or diminish the effect of beauty on the probability of boning.
Wealth: A rich observer has a better shot (see above).
Marriage: Marriage, in some cases, eliminates any possibility of boning. In other cases, it only makes the boning better.
Avon lady: If you are a creepy drug dealer, this can be a plus point.
Rufies: Rufies typically nail it down.
Beauty in the eye of the Bee Holder
Someone long ago said that beauty was in the eye of the tree folder, but since the art of tree folding was lost centuries ago the expression has gone through a historical game of telephone until it arrived at the current interpretation. Because of this, many people have had affairs with beekeepers (who, by nature, are lonely and easily seduced) to confirm (honestly or otherwise) their beauty. Others have tried to make an extract of beauty from the eyes of beekeepers, with limited success.