(1) Original usage: a yarn spun by hand in the Shetland Islands from the wool of sheep bred and reared in these islands.
(2) Common usage: a yam, spun on the woollen system from 100% new wool, of a quality capable of imparting to a fabric the handle attributed to the products formerly made exclusively from the Shetland breed of sheep.
(3) Current trade usage: (as recognised by the International Wool Textile Organisation) where the term Shetland is qualified by the adjective ‘genuine’, ‘pure’, ‘real’, or any similar description, implies that the wool actually originated in the Shetland Islands.
Textile Resource (http://www.textile.org.uk)
