Most Favorite Nation (MFN) is a negotiated provision in SAFE or convertible investments, which provides the investor with the best terms for that group of investments. This is sometimes used in early investments where other terms (valuation cap or discount) are not yet decided, and the investor and company want to execute the investment quickly.

Some MFN provisions require the investor be informed of new investments with better terms, then specifically electing to take the new terms.
Some MFN provisions apply on conversion, where the actual conversion to shares is determined based on the MFN and any other terms.