A CUSIP is a nine-character code that identifies a specific security issued in the US or Canada: one class of a company's shares, one bond issue, one fund. The first six characters identify the issuer, the next two the issue, and the last is a check digit.
Where you meet it
13F reports list each holding by its CUSIP, along with the issuer's name and the class of security. A company with two share classes has two CUSIPs.
On Listed
CUSIPs are licensed identifiers, and the master list is not public. Listed keeps no CUSIP master, so 13F holdings are shown as filed and are not linked to company pages. For the same reason, a holdings file you import must use tickers: a row with only a CUSIP stays unmatched.