iou

(=I owe you).

n.借据

I don't have the money so I'll give you an IOU. 我没有钱，所以会给你一张借据。

A bond, of course, is just an IOU, a promise to pay back money with interest by certain dates. 债券，当然，这只是一个借据，承诺到期归还并支付利息的纸条。