statistical mechanics

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

• noun Any of various statistical models used in physics, as in kinetic theory, to describe the aggregate properties of large numbers of particles.

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

• noun physics The branch of physics that uses statistical laws to make theoretical predictions about macroscopic systems of particles.

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

• noun the branch of physics that makes theoretical predictions about the behavior of macroscopic systems on the basis of statistical laws governing its component particles