# saddle

A saddle is a type of surface that is
neither a peak nor a valley but still has a zero gradient. Saddle points
are situated such that moving in one direction takes one uphill, while moving
in another direction would be downhill. A **saddle function** is a function *f *(*x*, *y*)
of two vectors *x* and *y* (which typically lie in different vector
spaces) that is concave up in *x* and concave down in *y*.