probability distribution A function of a discrete random variable (that is, a variable whose values are obtained from a finite or countable set) yielding the probability that the variable will have a given value. Also called probability density.
mathematical expression that describes the probability that a system will take on a specific value or set of values. The classic examples are associated with games of chance. The binomial distribution gives the probabilities that heads will come up a times and tails na times (for 0an), when a fair coin is tossed n times. Many phenomena, such as the distribution of IQs, approximate the classic bell-shaped, or normal, curve (see normal distribution). The highest point on the curve indicates the most common or modal value, which in most cases will be close to the average (mean) for the population. A well-known example from physics is the Maxwell-Boltzmann distribution law, which specifies the probability that a molecule of gas will be found with velocity components u, v, and w in the x, y, and z directions. A distribution function may take into account as many variables as one chooses to include.