Tail has two meanings in statistics. One is the long spread of low frequency values in a long-tail distribution, which includes very extreme values at the end(s) of the scale. The other is the use of the term to indicate in which directions the data is of interest – one, or both; see one-tailed test and two-tailed test.
Also used in hcistats2e: Chap. 4: page 49
Used in glossary entries: long-tail distribution, one-tailed test, two-tailed test