kidsneedscience:

The word (and concept) of billion is relatively new to human thought.  Sure the Ancient Greeks were predicting and naming numbers far larger than a billion, but in popular use the concepts of million and billion are relatively new to human history. A billion was just a really big number-very hard to imagine in real terms, an abstract notion that for most meant something like ‘too much to count’.  Etymologically, billion is a fabrication, a 15th century combination of the prefix bi- and the word million, itself a modern word.  Coming to English in the 1680s, from French mathematician Nicolas Chuquet who named a million million a byllion in his unpublished work Le Triparty en la Science des Nombres from 1484.  In England and Germany numbers were compiled in groups of sixes, later altered by the French into groups of three and becoming a thousand million, which is its current US meaning.  Chuquet was interested in naming huge numbers and devised the system of grouping by sixes, summarizing it this way starting with:  million, the second mark byllion, the third mark tryllion, the fourth quadrillion, the fifth quyillion, the sixth sixlion, the seventh septyllion, the eighth ottyllion, the ninth nonyllion and so on with others as far as you wish to go…

Carl Sagan made the notion and number both popular and accessible describing the size of the universe and the number of stars and galaxies in it.  Although famous for the phrase ‘billions and billions’, it wasn’t a phrase that Sagan had used by the time he became both known and parodied for it.  He later embraced it as a calling card, opening speeches and presentations with his signature ‘quote’.  A billion can be visualized as a cube of marbles 1000 marbles high 1000 marbles wide and 1000 marbles deep.  This cube would be approximately 40 feet to a side-and weigh many many tons.  

On October 31, 2011 the world population is estimated to have reached 7 billion, adding one billion people in just 42 years.