From Leopard Gecko Wiki
When a Leopard Gecko is homozygous for a trait, it means it has two copies of the gene. An example of a Leopard Gecko who is homozygous for a trait is an albino. Every Leopard Gecko who is an albino, no matter what the strand, it is homozygous for its strand.
When using the letters on the Punnent Square, a homozygous trait would be represented by either two lowercase letters who are the same or by two uppercase letters which are the same.