As there is no recognized breed standard for teacup Yorkshire terriers, the definition of these dogs is loose, including any dog from a Yorkshire lineage which is under 4 pounds at adult weight. Yorkshires bred to the AKC breed standard are between 4 and 7 pounds.
Adult dogs which are smaller than 4 pounds cannot be exhibited in dog shows as Yorkshire terriers, as they are outside the breed standard. This is to prevent the propagation of genetic health problems within the breed, to which smaller dogs are more prone. The Yorkshire terrier breed itself is considered a toy breed.
The term "teacup" is used to describe an adult Yorkie that weighs fewer than 4 pounds. However, according to breed standard, a Yorkshire Terrier typically weighs under 7 pounds and is considered a small breed dog regardless of its weight. Famous for her small size is Sylvia, an English Yorkie who was the smallest dog in recorded history as of 2014. She weighed just 4 ounces and stood 2.5 inches tall.