Key Points The X-bar is the sample mean of a given data set. ... In statistics, the Latin small letter x is used to represent a variable in a set of sample data.

Because it’s been employed in so many fields—from algebra, to genetics, to aerospace, to sex and spirituality—X's many meanings make it the most mystifying letter of the alphabet.

In a paper by Lord Viscount Brouncker in the Philosophical Transactions (vol. ii., 1668, p. 646), the capital letter X occurs regularly as the symbol for multiplication.

Because it’s been employed in so many fields—from algebra, to genetics, to aerospace, to sex and spirituality—X's many meanings make it the most mystifying letter of the alphabet.