I have been using the term hemizygous and heterozygous deletion interchangeably for the last few years to describe a single allele/copy deletion in cancer genomes. But it was recently brought to my attention that the term heterozygous deletion is not always correct when describing single copy deletions. The term heterozygous implies that the original two alleles of a genomic locus were different. But we may observe a single allele deletion where the original two alleles were identical. In this case, this would not be a heterozygous deletion, but rather it would be a hemizygous deletion which implies there is only copy remaining but makes no claim that the two original alleles were different.

So basically if you are describing a single allele/copy deletion, then it is always safe to call it a hemizygous deletion. You can only call it a heterozygous deletion if you are sure that the original two alleles were actually different from each other.