In the field of percolation theory, the term percolation threshold is used to denote the probability
which "marks the arrival" (Grimmett 1999) of an infinite connected component
(i.e., of a percolation) within a particular model.
The percolation threshold is commonly denoted and is sometimes called the critical phenomenon of the model.

Special attention is paid to probabilities both below and above the percolation threshold; a percolation
model for which
is called a subcritical percolation while a model satisfying is called a supercritical percolation. Because of this
distinction, the value is also sometimes called the phase transition of the model
as it marks the exact point of transition between the subcritical phase and the supercritical phase . Note that by definition, subcritical percolation models
are necessarily devoid of infinite connected components, whereas supercritical models
always contain at least one such component.

A great deal of literature has been devoted to the identification of the percolation threshold within a number of models and, indeed, nearly all literature which highlights a specific percolation model does so in order to study and present information related to that model's percolation threshold.

This concept is particularly well-studied in the case of discrete percolation theory on certain classes of "well-behaved" point
lattices. In this context, the percolation threshold is the fraction of lattice
points that must be filled to create a continuous path of nearest neighbors from
one side to another.

The following table is taken from Stauffer and Aharony (1992, p. 17). Entries indicated with an asterisk (*) have known exact solutions.

Essam, J. W.; Gaunt, D. S.; and Guttmann, A. J. "Percolation Theory at the Critical Dimension." J. Phys. A11,
1983-1990, 1978.Finch, S. R. "Percolation Cluster Density
Constants." §5.18 in Mathematical
Constants. Cambridge, England: Cambridge University Press, pp. 371-378,
2003.Grimmett, G. Percolation,
2nd ed. Berlin: Springer-Verlag, 1999.Kesten, H. Percolation
Theory for Mathematicians. Boston, MA: Birkhäuser, 1982.Stauffer,
D. and Aharony, A. Introduction
to Percolation Theory, 2nd ed. London: Taylor & Francis, 1992.