The SAFE Note is a  Simple Agreement for Future Equity - a legal document template facilitating streamlined investments in early-stage companies, reducing the need for legal process and expense. Current templates are maintained by YCombinator and available at https://www.ycombinator.com/documents
The SAFE was originally created by Adeo Ressi to streamline early-stage investing with a standardized document, replacing bespoke convertible notes or priced rounds. It was popularized by YCombinator.