Experience (also referred to EXP) is a term used in most games as well as The Last Story. EXP is used for leveling up your characters. To gain EXP, the character needs to defeat enemies. Depending on the level of the enemy, the character can gain more or less experience. For example: A Level 176 boss, gives 99,999 EXP, while a Level 87 monster only gives 36,000 EXP. The max EXP you can get is 99,999. However, you need at least more than a million EXP to reach Level 99.
Upon defeating an enemy, you would know that you gained EXP if you see green letters above your victory. Example: When you defeat a single Reptid in Reptid's Cave, you may see the green letters that say "EXP 18". After the battle, the player will see a small box that will pop up on their right side of their screen. It will show the current memebers of their party as well as if they leveled up or not.
Once a character reaches Level 99, that character can no longer gain EXP. When they fight, you can still see the green letters, but they will not count, since Level 99 is the maximum level a character can reach.