They are different definition of Augmented Reality (AR), simply does it means adding things like pictures, text, 3D-objects or sounds into the real world with a support device such as smartphones or special glasses. Wikipedia describes Augmented Reality in March 2016 as “a live direct or indirect view of a physical, real-world environment whose elements are augmented (or supplemented) by computer-generated sensory input such as sound, video, graphics or GPS data”. In this definition, the direct view of a physical world means to add objects to the front of the real-world environment. An example is the BMW colour Head-up Display in a Car.
Indirect view of a physical world means that there is a video or a live feed with added information (eg. Score by sports event on TV.)
In the Handbook of Augmented Reality  defined Julie Carmigniani and Borko Furht “Augmented Reality (AR) as a real-time direct or indirect view of a physical real-world environment that has been enhanced/augmented by adding virtual computer-generated information to it.” Compared to the Wikipedia definition, Carmigniani and Furht have not closer describe the computer-generated information. The goal of AR is simplifying the user’s life by bringing virtual computer-generated information in his immediate surrounding and add also an indirect view of the real-world environment like live-video streams said Carmigniani and Furht.
Cruz-Neira et al.  and Milgram, Kishino  have called the continuum that spans between the real environment and the virtual environment as mixed reality. The Mixed Reality has been split into the Augmented Reality (AR) close to the real environment and into Augmented Virtuality (AV) close to the virtual environment. This definition is known as the Milgram’s Reality-Virtuality (RV) continuum.
Augmented Reality is a physical real-world environment with virtual objects.
Augmented Virtuality is a virtual world merged with real objects.
The picture below shows the different between AR and AV. 
Nowadays the terms Augmented Reality, Mixed Reality and Enhanced Reality often used as synonyms for a presentation of additional information in a foreground.
Azuma et al.  says that AR has not to be considered as restricted to a particular type of technologies, nor is it limited to the sense of sight. AR can potentially apply to all senses, augmenting smell, touch and hearing.
 J. Carmigniani, B. Furht. (2011). Handbook of Augmented Reality. Springer: New York. S.3
 C. Cruz-Neira, D.J. Sandin, T.A. DeFanti. (1993). Surround-Screen Projection-based Virtual Reality: the Design and Implementation of the CAVE. Proceedings of the 20th Annual Conference on Computer Graphics and Interactive Techniques, S.135–142
 P. Milgram and F. Kishino. (1994). A Taxonomy of Mixed Reality Visual Displays. IEICE Transactions on Information Systems, Vol E77-D, No.12
 Ronald Azuma, Yohan Baillot, Reinhold Behringer, Steven Feiner, Simon Julier, Blair MacIntyre. (2001). Recent Advances in Augmented Reality