Core Mechanics

Roll

A roll is the act of physically or virtually rolling one or more dice with or without modifiers to determine the result of an action a creature takes.

Check

A check is a roll with a determined numerical goal, usually done with a 1d20.

Opposed Check

An opposed check is a check in which two opponents each roll 1d20 with one or more predetermined attribute MODs. The higher total wins.
In case of a tie, roll again.

Save

A save is a check to avoid a (usually negative) result of another creature’s action.

Determined Check (DC)

A determined check is a check, often a save, in which the target value is not determined by another roll, but instead a flat 10 plus the relevant attribute MOD or Quality value of the check's origin.

Advantage X

Roll X additional dice for the check. The highest result counts.

Disadvantage X

Roll X additional dice for the check. The lowest result counts.

Advantage Score

Sum of Advantage X and Disadvantage −X for a specific action.
If the Advantage Score is positive X: perform the action with Advantage X.
If it is negative -X: perform the action with Disadvantage X.

Reroll X

If a roll allows X Rerolls, that amount of dice may be discarded and rolled again.
The type of dice that may be rerolled is usually specified and may vary in scope. For example, a Damage Reroll only applies to damage rolls, while an Effect Reroll includes not just damage, but any other roll that results from the action.
If the type is not specified, assume an Effect Reroll.

Eased X

If a roll is Eased by X, add X to the result.

Hindered X

If a roll is Hindered by X, deduct X from the result.

Median Check

For a Median Check, roll three identical dice. Ignore the highest and lowest dice, only the middle value counts.
Example:
The results are 2, 11, and 17. The 11 is the result of the roll.
If a creature has Advantage on a Median Check, roll four or more dice. The second-highest result counts.
If a creature has Disadvantage on a Median Check, roll four or more dice. The second-lowest result counts.

Rounding

Numbers with fractional parts are always rounded down.

Explosion

On a maximum roll (e.g., 6 on 1d6) the die "explodes," and you may add another die of the same size to the rolled value.
If a maximum is rolled again, you may add an additional die of the next smaller size to the total. Dice smaller than 1d4 are not permitted.
Non-exploding dice: Some dice can not explode. This is either specified in the item, skill, or spell description, or signified by a different color, e.g. 1d20.