What is Allelic Imbalance
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.