Any non-random number added to or subtracted from a d20 roll. For example, your Attack Bonus is a modifer that is the sum of other modifiers. It can be helpful to think of modifiers as increasing or decreasing your chances of success by 5% increments. For example, if you need a 15 or higher on a d20 to score a hit, that means you have a 30% chance of success (15 to 20 = 6 chances out of 20 = 30%). If you increase your Attack Bonus by 2, you have added 10% to your chance of success (15 to 20 + 2 = 13 to 20 = 8 chances out of 20 = 40%). Similarly, if you decrease your Defense by 2, you have increased your chance of getting hit by 10%.