Billion is a word for a large number, and it has two distinct definitions:
- 1,000,000,000, i.e. one thousand million, or 109 (ten to the ninth power), as defined on the short scale. This is its only current meaning in English.
- 1,000,000,000,000, i.e. one million million, or 1012 (ten to the twelfth power), as defined on the long scale. This number, which is one thousand times larger than the short scale billion, is now referred to in English as one trillion. However, this number is the historical meaning in English for the word "billion" (with the exception of the United States), a meaning which was still in official use in British English until some time after World War II.
American English adopted the short scale definition from the French (it enjoyed usage in France at the time, alongside the long-scale definition). The United Kingdom used the long scale billion until 1974, when the government officially switched to the short scale, but since the 1950s the short scale had already been increasingly used in technical writing and journalism. 
Other countries use the word billion (or words cognate to it) to denote either the long scale or short scale billion.
Milliard, another term for one thousand million, is extremely rare in English, but words similar to it are very common in other European languages. For example, Bosnian, Bulgarian, Catalan, Croatian, Czech, Danish, Dutch, Finnish, French, Georgian, German, Hebrew (Asia), Hungarian, Italian, Kazakh, Kyrgyz, Lithuanian, Luxembourgish, Norwegian, Persian, Polish, Portuguese (although the expression mil milhões — a thousand million — is far more common), Romanian, Russian, Serbian, Slovak, Slovene, Spanish (although the expression mil millones — a thousand million — is far more common), Swedish, Tajik, Turkish, Ukrainian and Uzbek — use milliard, or a related word, for the short scale billion, and billion (or a related word) for the long scale billion. Thus for these languages billion is a thousand times larger than the modern English billion.