golden section

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A ratio, observed especially in the fine arts, between the two dimensions of a plane figure or the two divisions of a line such that the smaller is to the larger as the larger is to the sum of the two, a ratio of roughly three to five.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry the division of a whole into two parts such that the ratio of the smaller part to the larger is equal to the ratio of the larger part to the whole.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the proportional relation between two divisions of line or two dimension of a plane figure such that short : long :: long : (short + long)