Some definitions relating to theory of probability:
Experiment: Any process or procedure for which more than one outcome is possible. For example, toss a coin twice
The sample space S of an experiment is a set consisting of all of the possible experimental outcomes. For example, there are 4 possible outcomes of tossing a coin twice in an experiment. These are:
S = {Head, Head ; Head, Tail ; Tail, Head ; Tail, Tail}
Event: a subset of possible outcomes
A={HH}, B={HT, TH}
