Probability is that branch of Mathematics which deals with the **measure of uncertainty** in various phenomenon that gives several results or outcomes instead of a particular one.

**Random Experiment:** An experiment in which all possible outcomes are known but the results can not be predicted in advance.

**Trial:** Performing an experiment.

**Outcome:** Result of the trial.

**Equally likely outcomes:** Outcomes which have equal chances of occurrence.

**Sample space:** Collection of all possible outcomes. For example,

- Coin tossed once: S = {H, T}
- Coin tossed twice or two coins tossed simultaneously: S = {HH, HT, TH, TT}
- Die is thrown once: S = {1, 2, 3, 4, 5, 6}

**Event:** Collection of some including no outcome or all outcomes from the sample space.

**Probability of an event:** Number of outcomes favourable to the event divided by total number of outcomes in the sample space.

P(E) = n(E)/n(S)

**Sure Event:** Event whose probability is 1.

**Impossible Event:** Having no outcome or an event whose probability is 0.

**Range of Probability:** Probability of an event always lies between 0 and 1 ( 0 and 1 inclusive).

0 ≤ P(E) ≤ 1

**Complementary Event:** Event which occurs only when E does not occur.