ABR stands for Average Bitrrate and is basically just one form of Variable Bitrate where the encoder tries to maintain specific average bitrate for the file so that the actual size of the resulting file could be estimated more easily.
  
  To achieve a situation where you tell the encoder that you wish to use a specific ABR and that the encoder would actually do exactly that requires usage of multi-pass encoding technology. As there aren't currently any audio encoders available that can use multi-pass encoding, true respect-the-ABR-I-want-to type of encoding is possible only with video encoding.
  
  On the other hand, ABR can also simply state the average bitrate of a VBR encoded video or audio clip.