1,000,000,000 (one billion)

In the UK, this number used to be called, simply, 1,000 million, while a billion was reserved for a million million (a number with 12 zeros). But pressure to standardise our numbers with the US drove Harold Wilson to announce in 1974 that any government mention of a billion would from then on mean a number with nine zeros.

If you really want someone to blame for the confusion over billions, however, it's the French. Throughout history, they have flip-flopped between different definitions, wreaking havoc on the names of numbers. In 1480, they proposed that a billion have 12 zeros, which is what the British adopted. Then, in the middle of the 17th century, they knocked three zeros off, so a billion became a number with nine zeros. The young United States inherited this new definition. Then in 1948, the French reverted back to the old system.