Teacup is usually a term used to describe a puppy that will be in the 3 pound range or less at maturity, but it is not an official term. Many are extremely sensitive about it's use at all.
I have definitely seen "teacups" advertised that will more likely reach the high 4 and 5 pound range so check the puppy's weight and age and make a guess from that. Triple the 8 week weight and double the 12 week weight of a healthy puppy eating well should put you within a pound in most cases.
I have seen healthy 3 pounders, but I personally wouldn't want to buy one because the health risks are much higher. I prefer at least in the 4 pound range, but weight doesn't guarantee health.