To a geographer, I suppose it might be San Francisco, and to a dean, senior fellow. To a writer, SF means science fiction; but to a science fiction writer, SF (or s.f., or sf) can mean any of three overlapping genres – science fiction, speculative fiction, and science fantasy.

Needless to say, the name is not the thing named. You might write and sell all three without having any clear idea of the nomenclature involved and without having heard the terms; when Mary Wollstonecraft Shelley wrote Frankensteinback in the early years of the 19th century, she could not possibly have had any idea she was originating ‘science fiction’, which is a 20th century coinage. But if you understand how the three differ and know the basis of each, you will possess a considerable advantage.

Science fiction has the oldest name (although it is not really the oldest form), so we'll deal with it first. It may well be the hardest of the three to write; it is certainly the easiest to sell, when it is written competently.

Science fiction is fiction that turns upon the assumption of at least one breakthrough in one of the ‘hard’ sciences. (A hard science is one in which theories are subject to rigoroustest, usually by experiment. Physics, chemistry, biology, and mathematics are the principal hard sciences.) Mary Shelley, for example, extrapolated from experiments in galvanism and assumed, for the purposes of her book, that the time would soon come when life could be restored to dead flesh. In other words, she assumed a certain breakthrough in biology or biochemistry. When H. G. Wells assumed that a machine capable of travelling through time could be built, he was assuming a breakthrough in physics.

In both of those books, ‘we’ – members of western civilisation – were supposed to have made the breakthrough; but it doesn't actually matter who makes it. Wells's The War of the Worldsassumes that Martians develop interplanetary flight and come to Earth; it is just as much science fiction as Stanley Weinbaum's ‘A Martian Odyssey’, in which Americans go to Mars.

Too often, would-be science fiction writers suppose that any story that has a rocket ship, a ray gun, or a robot in it is science fiction, and that any story that lacks all three is not.