A grade of extremely fine, tightly spun white linen thread. It earned its name because it was spun by nuns in European convents specifically for crafting delicate lace and ecclesiastical embroidery. The high twist and smooth finish made it strong enough for intricate needlework without fraying. Equivalents are usually mercerized cotton rather than true linen.
Heard on the job
You'll need something as fine as nun's thread if you want to repair that antique lace collar without the stitches showing.
Insider tip
True linen nun's thread is rare, so restorers often substitute a 100-weight silk or fine Egyptian cotton for antique repairs.