What is Allelic Imbalance Can anyone help me explain what allelic imbalance is, hopefully shortly? 
Surprisingly, we cannot find any introduction online.
It might be used differently in different contexts, but generally speaking, in my world - allelic imbalance is when there's a difference in the level of gene expression from different alleles, usually through genetic (e.g. a variant in a promoter) or epigenetic mechanisms (e.g. one copy silenced, as in imprinted regions).

There are a few blog posts here with some more information, and plenty of papers (google "Allelic imbalance") if you want to dig deeper.