complexity theory

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

• noun Any of various branches of mathematics, physics, computer science, and other fields concerned with the emergence of order and structure in complex and apparently chaotic systems.

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

• noun computer science The study and classification of decision problems by the computational resources—such as time and space—needed by the programs that solve the problems.