I always knew that "Glat Kosher" Kashrut classification relates only to beef, it's when the cow is cooked and eaten within 24 hours of it being butchered. Recently I was told several times that there is "Glat Kosher" in chicken as well. Is that true? What is the definition of "Glat Kosher" for chicken? what is the base for it?
The short answer -- as indicated by others, technically "Glatt" refers to how smooth the lungs are. With chickens, any question of that sort and the bird is not kosher. So there is no such thing as "Glatt" chicken.
There are, however, many other halachic preferences that will distinguish a "basic kosher" product from an "extra kosher" or "mehadrin" product; because "extra kosher" beef will be "glatt", people have started using that term for any kosher preference.
One such preference that applies to both beef and chicken -- and perhaps this is what you had in mind -- is that the soaking and salting be performed within 72 hours of slaughter, rather than stopping the clock by freezing the meat or re-washing it.
Other preferences that you'd find in an "extra-kosher" meat product (beef or poultry) pertain to how many animals the slaughterer does between breaks.
Glatt is about the lungs, not the timing (AFAIK) and it doesn't apply to chicken.
Misconception: "Glatt Kosher" means something like "extra kosher" and applies to chicken and fish as well as meat.
Fact: Glatt is Yiddish for smooth, and in the context of kashrut it means that the lungs of the animal were smooth, without any adhesions that could potentially prohibit the animal as a treifa, an issue only applicable to animals, not fowl or non-meat products.
For other info, try here.