Well, sure, but it's not a hard line. Over time, the offshoot that became chickens (gallus gallus) would have been less and less capable of breeding successfully with other members of the parent population (gallus) due to random combinations of genes, and some combinations being less viable.
Then you reach the point for things like horse+donkey, where a mule winds up being sterile. This is where you start to declare separate 'species', but this may still not apply to the whole population on either side just yet.
Our nomenclature for things like "Horse" or "Chicken" is simply a useful tool to describe a population of specific sets of genes. What happens if you geographically separate the chicken population into groups for a long time? Speciation. So which group do you still call 'Chickens'? Both? But if they're separate species, how do you call them Chickens?
So, to sum this lot up. Chicken is purely a contemporary term. There's no definable point at which you can go back and say "not a chicken", since you'd be playing the same game that creationists play when they make an arbitrary division between man and ape when looking at fossilized remains. There is no useful point at which you can go back and say definitively, "Not a chicken". You can only go back and say "member of ancestor species", because they could still inter-breed.