Merriam-Webster Collegiate Dictionary - t distribution

 noun  Date: circa 1957 a probability density function that is used especially in testing hypotheses concerning means of normal distributions whose standard deviations are unknown and that is the distribution of a random variable t = (uvn)/v where u and v are themselves independent random variables and u has a normal distribution with mean 0 and a standard deviation of 1 and v2 has a chi-square distribution with n degrees of freedom — called also student's ~