X-factor

Never mind the TV talent show that borrowed the term in the early 21st century, and defined the x-factor as that special quality that leads to stardom as a singer or performer. The x factor derives originally from algebra, where x has always signified an unknown quantity. However, the expression 'x-factor' meaning an unknown or inexplicable talent or proficiency at something or other, is first cited from the 1930s, according to Merriam-Webster. Apart from this 1930s date, its origin, or who first used the expression in this context, is unknown.