The web site of the International Society for Augmentative and Alternative Communication (ISAAC) defines Augmentative and Alternative Communication (AAC) as:

‘the term used to describe methods of communication which can be used to add to the more usual methods of speech and writing when these methods are impaired.
‘AAC includes unaided systems such as signing and gesture, as well as aided techniques ranging from picture charts to the most sophisticated computer technology currently available. AAC can be a way to help someone understand, as well as a means of expression.’

You can get more information about AAC from the ISAAC web site. Our book, Augmentative Communication in Practice: An Introduction provides a good starting point for somebody new to the field. You can download individual chapters from here.