Julia set

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

• noun For any holomorphic function on a complex plane, the boundary of the set of points whose result diverges when the function is iteratively evaluated at each point.

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

• noun In the mathematical context of complex dynamics, a set consisting of values generated by a mathematical function such that an arbitrarily small perturbation can cause drastic changes in the sequence of iterated function values.