A clause in a SAFE that entitles the holder to adopt any more favorable terms given to later investors during the same fundraising campaign.
The Most Favored Nation (MFN) clause in a SAFE means that if the company issues another SAFE to a subsequent investor on better terms, the existing MFN holder can elect to amend their SAFE to match those better terms. It protects early investors from being disadvantaged by terms the company later gives to investors who arrive after them.
MFN clauses are typically found in uncapped SAFEs (SAFEs without a valuation cap). The logic is: since the investor took early risk without a cap, they should at least be protected if the company gives a cap to the next investor. An MFN lets them adopt the lower cap retroactively.
From a founder perspective, MFN clauses add complexity because you have to track all existing SAFE holders and notify them when you issue new SAFEs on different terms. It also limits your flexibility to give favorable terms to late investors without those terms flowing back. For simple seed raises, many founders avoid MFN clauses by giving all investors a cap from the start.