Percolation threshold  The percolation threshold is a mathematical concept in percolation theory that describes the formation of long-range connectivity in random systems. Below the threshold a giant connected component does not exist; while above it, there exists a giant component of the order of system size. In engineering and coffee making, percolation represents the flow of fluids through porous media, but in the mathematics and physics worlds it generally refers to simplified lattice models of random systems or networks (graphs), and the nature of the connectivity in them. The percolation threshold is the critical value of the occupation probability "p", or more generally a critical surface for a group of parameters "p"1, "p"2, ..., such that infinite connectivity ("percolation") first occurs. The most common percolation model is to take a regular lattice, like a square lattice, and make it into a random network by randomly "occupying" sites (vertices) or bonds (edges) with a statistically independent probability "p". At a critical threshold "pc", large clusters and long-range connectivity first appears, and this is called the percolation threshold. Depending on the method for obtaining the random network, one distinguishes between the site percolation threshold and the bond percolation threshold. More general systems have several probabilities "p"1, "p"2, etc., and the transition is characterized by a "critical surface" or "manifold". One can also consider continuum systems, such as overlapping disks and spheres placed randomly, or the negative space ("Swiss-cheese" models). In the systems described so far, it has been assumed that the occupation of a site or bond is completely random—this is the so-called "Bernoulli percolation." For a continuum system, random occupancy corresponds to the points being placed by a Poisson process. Further variations involve correlated percolation, such as percolation clusters related to Ising and Potts models of ferromagnets, in which the bonds are put down by the Fortuin-Kasteleyn method. In "bootstrap" or "k-sat" percolation, sites and/or bonds are first occupied and then successively culled from a system if a site does not have at least "k" neighbors. Another important model of percolation, in a different universality class altogether, is directed percolation, where connectivity along a bond depends upon the direction of the flow.