AI is an acronym for Artifial Intelligence.

It is a broad covering definition of the concepts that may make a machine appear to be or actually be behaving intelligently.
There are many techniques which fall under this banner

Artificial Life Genetic Algorithms
Neural Networks
Natural Language Processing
Subsumption Architecture
Expert Systems

A system does not necessarily have to learn to be considered AI, although it is considered closer to being truly intelligent behaviour if it can learn.