The earliest known usage of the term “science fiction” is in 1851 (in Chapter 10 of William Wilson’s A Little Earnest Book upon a Great Old Subject), in which he writes: “Science-Fiction, in which the revealed truths of Science may be given interwoven with a pleasing story which may itself be poetical and true.”

However this appears to be an isolated usage and the term appears to have been recoined in the 1920s where it appeared in Amazing Stories.

Science fiction is often abbreviated as SF or sci-fi. However, SF is not unambigous (see under Other types, below), and sci-fi is seen by some as having derogatory overtones. A short lived synonym was scientifiction. (via Wikipedia)