This is the Target Number that an attacker must equal or beat in order to make a physical attack against your character. Your character’s Base DEF is comprised of several factors as shown here:

DEF = 15 + Spd Bonus + (1/5 ranks melee Combat Skill)

The base of 15 and the Speed Stat Bonus are pretty much self-explanatory. For every 5 ranks that the character has in his best melee-based Combat Skill, he gains a +1 modifier to his DEF. This means that when he gets 5 ranks in a melee skill, he gains a +1, when he gets 10 ranks; he gets another +1, for a total modifier of +2, and so forth.

There are other things, such as special abilities, shields, spells, Combat Moves, and Boon Point results that can alter your character’s DEF for either good or bad. The majority of these are situational, meaning that they only apply in certain situations.

If your character is prone (laying on the ground), caught completely unawares (which is different from just being surprised) or unable to move for any reason, his DEF would be 12 + any magical bonuses.