Represents how well your mobile suit can protect itself from damage dealt by melee weapons, e.g Beam Sabers, Heat weapons, Tackles. It should be noted that raising your Beam Defense stat does not lessen the damage taken from beam type melee weapons, such as Beam Sabers, all damage taken from melee weapons will be affected by the melee defense stat alone.
Melee Defense is the fourth listed stat from the top of your mobile suit's list of stats.