"Billion" is a word which is somewhat ambiguous in U.K. practice, but not in American. For several centuries in Britain, it meant "a million million" (1,000,000,000,000 = 10^12 = US trillion), but in 1974, the British government announced that it would use "billion" to mean 10^9, the same as US usage. For the last several decades, English-language publishers have used "billion" or "thousand million" for 1,000,000,000 = 10^9 = US billion.
The "old" use is still encountered just often enough (in speech, informal writing, and older books) to cause some Britons to be unsure of what the speaker or writer means by the word.
The first few U.S. words for large numbers, and the corresponding traditional British terms, are as follows:
U.S. Traditional British
10^6 million million
10^9 billion thousand million or milliard
10^12 trillion billion or million million
10^15 quadrillion thousand billion
10^18 quintillion trillion
Scientists have long preferred to express numbers in figures rather than in words, so it is easy to avoid "billion" in contexts where precision is required. The plural, "billions", is still used freely with the colloquial meaning of "a very large number".
Some articles with more history about these terms: