The class of company shares issued to venture investors at each priced round, carrying liquidation preference , anti-dilution protection, board rights, and other contractual privileges that common stock does not have.
When Halsey Minor and Larry Ellison led Salesforce's Series A, the financing used the standard Wilson Sonsini preferred-stock model: 1x non-participating liquidation preference, weighted-average anti-dilution, and a protective-provisions list covering future financings and changes of control. That structure became the default that every YC and Stanford-trained founder still signs in some form today.