The computer security code printed on the back of Visa credit cards. The CW is actually the three letters C-V-V, but looks like "CW" because proportional fonts squash the two V's together. See CSC.
"CW number." YourDictionary, n.d. Web. 12 August 2018. <http://www.yourdictionary.com/cw-number>.
CW number. (n.d.). Retrieved August 12th, 2018, from http://www.yourdictionary.com/cw-number