The radius which a spherical mass has to be compressed to transform it in a black hole, or the radius at which space and time switch responsibilities. Once inside the event horizon, it is impossible fundamentally to escape to the outside. In addition, nothing can prevent a particle from hitting the singularity within extremely short amount of proper time once it has entered the horizon. In this case, the event horizon is a "point of no return."
For generalized black holes (in geometrized units) the radius of the event horizon, r, is
r = m + (m2 - q2 - s/m2)1/2,
Where m refers to the mass of the hole, q refers to its electric charge, and s refer to its angular momentum.