1) Genes which result in the premature death of the organism; dominant lethal genes kill heterozygotes, whereas recessive lethal genes kill only homozygotes.

2) Any gene in which a mutation will result in the premature death of the organism carrying it.

3) A gene that in some (as homozygous) conditions may prevent development or cause the death of an organism or its germ cells -- called also lethal factor, lethal mutant, lethal mutation.