Heterozygous means a pair of non-identical alleles. Alleles are different versions of the same gene. The usual case is a normal allele paired with a changed (mutant) allele. An individual that is heterozygous for a recessive mutant gene look normal.
One copy of an allele. Typically shortened to “het” and often refers to recessive genes – however, technically all incomplete dominant genes are heterozygous for their ‘super’ form (so mojave is ‘het super mojave’ technically).