The numeral billion refers to either of two different numbers.
10^12
The original meaning, since the beginnings of modern times, is "a million of a million" (= 10^12). This system, known in French as the échelle longue ("long scale"😉 was maintained or readopted all around the world—with the exceptions of the USA and Brazil
10^9
In the late 17th century a change was made in the way of writing large numbers. Previously, numbers were separated into groups of six digits; subsequently, the modern grouping of only three digits came into use. Then, a minority of scientists, particularly in Italy and France, changed the original meaning of the number billion to be equal to 10^9. Under the influence of these French advisers, and to be distinguished from British use, the USA adopted this reformed meaning during the 19th century. Currently this meaning is the preferred meaning in financial world, and therefore most English-speaking countries, such as the United Kingdom, South Africa, and Australia, now use this value, known in French as the échelle courte ("short scale"😉.