A Convertible Note

A convertible note is legally a loan, with the full amount converted to preferred shares when the startup raises its Series A round. 

As a loan, the convertible note has a maturity date prior, typically 18–24 months, by which time the startup must raise the Series A round, or at least in theory, is on the hook for repayment. (Since repayment is impossible, an extension is almost always negotiated.)