When a particle decays, it often can decay in several ways. The likelihood of it decaying to a particular mode is known as its branching ratio or branching fraction for that decay mode.
ratio of the branching fractions for two specified modes of disintegration
<radiobiology> In a fusion reaction involving two nuclei, there are typically a variety of possible sets of products which can form. The branching ratio for a particular set of products is the probability that that set of products will be produced. ... (09 Oct 1997) ...