Definition of non-metals: The elements which form negative ions by gaining or accepting electrons are called non-metals.
Lustrous: Lustrous elements mean they have luster which means they have a shining surface.
Non-metal elements are usually non-lustrous except for Iodine(I).
Iodine(I) is a non-metal which is lustrous. Because Iodine(I) has a shiny surface.It possesses a metallic luster as iodine atoms exist in a three-dimensional crystal structure and reflect most of the light upon it.