If you are American, it is undoubtedly 1,000,000,000. This amount is known to traditionally minded British people as a thousand million and by some more adventurous ones as a milliard, though this word has not made as much headway in English as in some other European languages. A trillion is then 1,000,000,000,000, and so on.
If you are British, on the other hand, a billion may be 1,000,000,000,000 (a million million) following the older convention.
If you are neither British nor American, you can take your pick! Both systems were invented by the French but are called British and American for convenience.
Once the business world and the financial press found themselves discussing thousand millions so much, the American' system simply became more convenient, despite a certain lack of logical tidiness. A British trillion is the third power of a million while the American one is the fourth power of a thousand and the American system continues out of sync with the arithmetic. It also makes the profits sound bigger! The American system is now standard use in British government publications and is becoming the norm in many other languages.
1012 : trillion - American 1012 : billion - British
1015 : quadrillion - American 1015 : thousand billion - British
1018 : quintillion - American 1018 : trillion - British
1021 : sextillion - American 1021 : thousand trillion - British
1024 : septillion - American 1024 : quadrillion - British
1027 : octillion - American 1027 : thousand quadrillion - British
1030 : nonillion - American 1030 : quintillion - British
1033 : decillion - American 1033 : thousand quintillion - British
In the British system, it would also possible to use billiard for thousand billion, trilliard for thousand trillion, etc., but this has not so far caught on.