With deterministic experiments, the results can be predicted before the experiment is conducted.
If a stone is dropped from a window, it is know, undoubtedly, that the stone will go down. If it is throw up in the air, it is know that it will travel upwards over a certain time interval; but afterwards will come down.
With random experiments, the results cannot be predicted, since they depend on chance.
If a coin is flipped, it is not known beforehand whether it will be heads or tails.
Similarly, if a die is rolled, the result cannot be determined beforehand.
Probability theory deals with assigning a number to each possible result that can occur in a random experiment. With this being said, the following definitions need to be introduced: