In "Variation under Domestication", Darwin makes several references to the concept of true breeding:
They believe that every race which breeds true, let the distinctive characters be ever so slight, has had its wild prototype.
I crossed some white fantails, which breed very true, with some black barbs.
Before diving into the book, I never heard of the term, so I looked it up and found this definition in the Wikipedia article on Zygosity:
A cell is said to be homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell or organism in question is called a homozygote. True breeding organisms are always homozygous for the traits that are to be held constant.
But this is rather technical and it's not the kind of explanation I would give to a child.
What is true breeding, in simple terms? When is breeding not considered true?