From Glossary of Meteorology
(Also called t test.) A significance test applicable to a sample mean of a sample estimate of a regression coefficient or a correlation coefficient (the latter only when the hypothetical value is zero).
It is used to test an assumed value of the corresponding parameter. It is also applicable to the comparison of two (but not more than two) sample means or two regression coefficients, but not of two correlation coefficients. The t test has different formulas for different uses.