# Directional derivative

### From Glossary

The limit (if it exists) of the rate of change along a specified direction, say ,

In particular, the -th partial derivative is with . (Note
that the directional derivative, as defined above, depends on the
scale: the derivative for is times the derivative for . To
avoid scale dependence, some authors require .) Recently,
some people have called this a *B-derivative* (or *Bouligand
derivative*), and functions that have directional derivatives in
all feasible directions are *B-differentiable*. Some require the
convergence to be uniform.