"Teacup" is a term applied to dogs belonging to the toy group which are very tiny in size, even for their own breed. The word is a marketing gimmick and is not officially recognized as a breed description.
Before you decide you want to get a "teacup" Chihuahua or any other toy breed labeled as "teacup", be aware a toy dog that is undersized for its breed very often has health issues which may have an impact on how long it will live. It is also very fragile and prone to accidents, a fall from a bed or sofa, or even getting stepped on can be fatal.
Many breeders of small dogs advertise their litters as teacups because people are willing to pay more for them but no breeder can guarantee how large or small a puppy will grow. Those of you in search of a teacup would do well to keep in mind that even a toy dog that grows larger than it should be is still going to be a very small dog....and more fun to play with because you won't feel like you're walking on egg shells.