Mathworks defines "Deep Learning" as teaching e.g. artificial neural networks not with sets of numbers (prepared by the user), but with electronic files containing images, sounds, texts etc.
According to the Deep Learning book by Ian Goodfellow and Yoshua Bengio :
The hierarchy of concepts enables the computer to learn complicated concepts by building them out of simpler ones. If we draw a graph showing how these concepts are built on top of each other, the graph is deep, with many layers. For this reason, we call this approach to AI deep learning.
Deep learning resolves problem difficulty by breaking the desired complicated mapping into a series of nested simple mappings, each described by a different layer of the model.