Hellow aabid sir,
AU means Amplitude Units. It is the unit of representing Eye height. Eye Amplitude, measured in Amplitude Units (AU), is defined as the distance between two neighboring eye levels. For an NRZ signal, there are only two levels: the high level (level 1 in figure) and the low level.
http://in.mathworks.com/help/comm/ref/commscope.eyediagram.html?refresh=true