Definition of "Sagan"

Today, there is a well-known joke unit of measurement called a Sagan. a Sagan is four billion—the smallest number that could fit the description "billions and billions."

2014, Gabrielle Borisovna, Carl Sagan

I had resolved to pass along the eastern shore of Lake Ganjule, in order to solve the problem of the sources of the river Sagan, the largest affluent of Lake Stefanie, which was supposed to flow out of Lake Ganjule. This I found to be not the case. The sources of the Sagan lie east of the south end of Lake Abaya. But there is a broad channel connecting Lake Ganjule with the Sagan.

1904, Oscar Neumann, From the Somali Coast Through Southern Ethiopia to the Sudan‎, U.S. Government Printing Office, page 784