Lavinia Cohn-Sherbok and Dan Cohn-Sherbok define a Tallit Katan as a sacred undergarment for Orthodox Jews.
Lavinia Cohn-Sherbok and Dan Cohn-Sherbok, A Popular Dictionary of Judaism, (Milton Park: Routledge, 1995), 144
TALLIT KATAN (Hebrew, 'Small tallit') A garment with fringes at each corner. The tallit katan is worn by *Orthodox *Jews under their secular clothes. It is to fulfil the commandment of *Zitzit (see *Numbers 15) and it is worn with the fringes visible.