# 9.9. GlossaryΒΆ

**Box-Counting:** A way of determining the fractal dimension of a set S in a Euclidean space Rn, or more generally in a metric space (X, d).

**Critical Phenomena:** A wide variety of systems display a common set of behaviors and characteristics when they are at or near a critical point.

**Diffusion:** The intermixing of two or more substances by the natural movement of their particles.

**Fractal Geometry:** For example, freezing water tends to form fractal patterns, including snowflakes and other crystal structures. Fractals are characterized by self-similarity; that is, parts of the pattern are similar to scaled copies of the whole.

**Percolation:** A process in which a fluid flows through a semi-porous material.

**Percolation Model:** A model used to represent percolation that can be abstracted to be use for epidemic modeling, networks of electrical resistors, and other systems that are not directly percolation.

**Phase Change:** The rapid change in behavior near the critical value.

**Random Walk:** A stochastic process consisting of a sequence of changes each of whose characteristics (as magnitude or direction) is determined by chance.