I know there's some debate on exactly when "science fiction" itself became a genre, but I'm curious if anyone knows either 1) when the term science fiction was coined and 2) when sci-fi became accepted, too.
The Oxford English Dictionary notes usage of "Science-Fiction" going back to 1851 in William Wilson's A Little Earnest Book Upon A Great Old Subject
They also note that Britannica's 1955 Book of the Year mentions the contraction Scifi taking hold, demonstrating the popularity of the genre.
You can have statistics on the terms usage from google n-gram viewer. You can see here and plot below that science-fiction started to be really used in the 1950s (roughly as often as nowadays neutrino). .
Using the same tool, you can see that scifi and sci-fi are much less used (by a factor 100 at least), and start to be really seen only in the 1970s (here)
Edited to take capitalised version of the words into account
Somebody has to mention Hugo Gernsback here, so I guess it's me.
Gernsback started the modern genre of science fiction by founding the first magazine dedicated to it, Amazing Stories, in 1926. He said he became interested in the concept after reading a translation of the work of Percival Lowell as a child. His idea of a perfect science fiction story was “75 percent literature interwoven with 25 percent science.” He also played a key role in starting science fiction fandom, by publishing the addresses of people who wrote letters to his magazines. So, the science fiction fans began to organize, and became aware of themselves as a movement, a social force; this was probably decisive for the subsequent history of the genre. He also created the term “science fiction”, though he preferred the term “scientifiction”.
The Science Fiction Achievement awards, given to various works each year by vote of the members of the World Science Fiction Society, are named the “Hugos.” He was one of 1996's inaugural inductees into the Science Fiction and Fantasy Hall of Fame.
In 1960 he received a special Hugo Award as “The Father of Magazine Science Fiction.”
Gernsback's New York Times obituary described him as “the father of modern science fiction.”