The tail of a vector 
is the initial point 
, i.e., the point at which the vector originates.
The tails of a statistical distribution with probability density function 
are the extreme values of 
for which 
is less than some given threshold.
Tail
See also
Direction, Head, Skewness, Statistical Distribution, VectorExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Tail." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Tail.html