Determining the optimal measurements to make under a cost constraint. A measurement is "optimal" when it is expected to give the most new information about the parameters of a model. Active learning is thus an application of decision theory to the process of learning. It is also known as experiment design.
Artificial Neural Networks: Ripley Chapter 5
Duda, Hart, and Stork Chapter 6
Nueral networks are a family of function approximation techniques, when the function is approximated,
$ f:x \rightarrow z $
is modeled as a composition of simple functions $ f_i $'s
$ f=f_n\bullet f_{n-1}\cdots f_1 $
