Why is there 2 definitions of 1 billion?

Answer 1

There were two different numerical naming systems. One (the "short scale" system) had the ratio between successive new number names as 1000. The other (the "long scale" system) had it at 1,000,000.

I guess it was reasonable (for people that used the long system) to say "thousand million" (10^9) and not feel like it needed a new name. So the next name (billion) wasn't used until the number that was a "million million" (10^12). Then you would have a thousand billion (10^15) and only a million billion (10^18) would be a "trillion."

The short system (what everyone uses today) obviously preferred short names to long names and its advocates probably felt that "thousand million" was too wordy... so that became "one billion"

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