A zygotic lethal gene is mutated or otherwise defective gene that doesn't interfere with the normal functioning of the eggs or sperm (gametes) that contain it, but which will kill the developing organism in its embryonic or larval stages.
In the nematode C. elegans, mutations to the gene pha-1 are lethal because this gene codes for the development of the pharynx and other organs.
In humans, the gene that codes for achondroplasia becomes a zygotic lethal gene if the embryo inherits two copies of the gene.
Some of this writeup is based on work I did for the science dictionary at http://biotech.icmb.utexas.edu/