A hit die is in simplest terms a 'title' given to any die used in calculating damage in pen and paper, and some digital RPGs that draw their roots from pen and paper. Its a little more complex in that its often used to mean "the hit die are" where you have a written structure of rolls of die, written as:

rolls d sides
```so:
the hit die  is as example  d20
the hit die are as example 4d20
```
The nature of this structure is allowing to have a range of randomness other than 1 to x, instead y to x, and also to effect the probability of achieving either the lowest or highest value due to your combination sets.

In the above example since your rolling four 20 sided die you can't achieve less than 4, and can achieve as much as 80, and the odds of achieving 4 or 80 are 20^4th ( 1 in 160k ) instead of with one 80 sided die 1 in 80, whereas mid range numbers grow much more probable with multiple die.

```4d2 probability chart:

d1| d2| d3| d4| Result
1 | 1 | 1 | 1 |    4                  1/16   =   4             6.25%
1 | 1 | 1 | 2 |    5                  4/16   =   5            25.00%
1 | 1 | 2 | 1 |    5                  6/16   =   6            37.50%
1 | 1 | 2 | 2 |    6                  4/16   =   7            25.00%
1 | 2 | 1 | 1 |    5                  1/16   =   8             6.25%
1 | 2 | 1 | 2 |    6
1 | 2 | 2 | 1 |    6
1 | 2 | 2 | 2 |    7
2 | 1 | 1 | 1 |    5
2 | 1 | 1 | 2 |    6
2 | 1 | 2 | 1 |    6
2 | 1 | 2 | 2 |    7
2 | 2 | 1 | 1 |    6
2 | 2 | 1 | 2 |    7
2 | 2 | 2 | 1 |    7
2 | 2 | 2 | 2 |    8
```