In nuclear and high-energy physics, a cross-section is a measure of the likelihood of a given process occurring in an accelerator, i.e., of particles colliding or otherwise interacting. Cross-section is expressed as an effective target area, which can be related to the quantum-mechanical probability of interacting by multiplying by factors such as the flux of particles entering the interaction region. The basic unit of cross-section for particle physics is the barn (b), equal to 10^-24 square centimeter (cm²). Typical hadron collisions are measured in millibarns. Typical neutrino collision cross-sections are about 10^-11 cm².