  The paper discusses the characteristic properties of fractional derivatives
of non-integer order. It is known that derivatives of integer orders are
determined by properties of differentiable functions only in an infinitely
small neighborhood of the considered point. Therefore differential equation,
which is considered for this point and contains a finite number of
integer-order derivatives, cannot describe nonlocality in space and time. This
allows us to propose a principle of nonlocality for fractional derivatives. We
state that if the differential equation with fractional derivative can be
presented as a differential equation with a finite number of integer-order
derivatives, then this fractional derivative cannot be considered as a
derivative of non-integer order. This means that all results obtained for this
type of fractional derivatives can be derived by using differential operators
with integer orders. To illustrate the application of the nonlocality
principle, we prove that the conformable fractional derivative, the
M-fractional derivative, the alternative fractional derivative, the local
fractional derivative and the Caputo-Fabrizio fractional derivatives with
exponential kernels cannot be considered as fractional derivatives of
non-integer orders.
