It's short for "heterozygous". It means carrying an unmatched pair of whatever genes you are talking about. For example, at the spider gene location a spider has a copy from one parent with the spider mutation and a copy of from the other parent without the spider mutation (i.e. normal for spider). The term homozygous is used when the pair matches (like homozygous normal or homozygous mutant with either two normal or two mutant copies respectively at the location you are talking about).