probability
Probability is a number that pins down how likely something is, on a fixed scale from 0 to 1. Zero means it simply cannot happen; 1 means it is certain; one-half means a coin-flip, equally likely either way. A fair die landing on a six sits at 1/6 — about 0.17 — a small but real chance you can feel in your bones every time you roll.
Why trust a single number to capture luck? Because over the long run it comes true. Flip a fair coin ten times and you might get seven heads; flip it ten thousand times and the share of heads creeps remarkably close to one-half. Probability is the promise hidden in that drift — the steady frequency a thing settles into when you give chance enough room to average out.
The common trap is the 'gambler's fallacy': after five reds at roulette, black feels overdue. It isn't. The wheel has no memory, and each spin carries the same odds as the first. Probability describes the long crowd of trials, never a debt that the next single try is forced to repay.
A fair coin: heads has probability one-half. Every probability lives between 0 (impossible) and 1 (certain).
People reasoned about odds for centuries, but probability only became rigorous mathematics in 1933, when the Russian mathematician Andrey Kolmogorov gave it three short axioms: probabilities are never negative, the certain event has probability 1, and the chances of mutually exclusive events add up. Almost all modern statistics rests on those three lines.