Split is short for a bird whose color genetics - in this case we're talking about gold or silver or a bird that carries both gold AND silver genes - that bird is SPLIT gened. And phenotypical (what it looks like) that bird will have sort of muddled, washed out silver and gold feathering, where a Gold bird would have the true golds, and a Silver would have good clean silver.
It's a bit of problem in the chicken world that Brown Red is the designation for Gold Birchen, in some breeds, and other names in other breeds, with the one word - BIRCHEN usually implying a Silver Birchen bird. But genetics can throw more curves - the bird I showed is a demelanized Brown Red (Gold Birchen) which is why where he should be black he looks Gray or Blue. He throws Split birchen and silver birch blacks. So he's black, with a demelanizer of some sort.
There are many common splits, split for birchen, splits for black/lavendar is common in someone working with lavendar. In some colors it shows, in others it's hidden and only test breeding shows the presence of the hidden or split gene.