Having more than one kind of sex chromosome (or gamete)
In humans, males are naturally heterogametic because they possess two types of sex chromosome: X-chromosome and Y-chromosome.
In gamete production, the males produce spermatozoa, each containing one sex chromosome, which may either be X or Y-chromosome. Thus, they also have two types of gametes: a sperm carrying the X-chromosome and another with the Y-chromosome.
Word origin: Greek héteros (other, different)