A definition of science fiction as to be found in a dictionary is "fiction often based on future or recent scientific discoveries, and dealing with imaginary worlds, space travel, or life on other planets."
But this definition is certainly very superficial and extremely summarizing, so I would like to extend the brief explanation with some more details by famous science fiction writers and publishers. What has to be mentioned first is the complexity of science fiction and why any definition will find its enemies:
"Ich nehme an, die Fruchtbarkeit des Science Fiction-Feldes sieht man unter anderem daran, daß sich keine zwei Schreiber des Genres auch nur auf etwas so Fundamentales wie seine Definition einigen können - oder auf seine Begrenzung - und wo man die Linie zwischen Science Fiction und realistischer Fiktion oder zwischen Science Fiction und Fantasy ziehen soll." (Asimov, "Meine Betrachtung der Science Fiction")
Translation: "I presume that the fertility of the SF genre can be realized by considering that no two writers can agree on such a fundamental thing as a definition - or a limitation - and where to draw the line between Science Fiction and real fiction or between Science Fiction and Fantasy." (Asimov, "My view of Science Fiction")
But some important aspects can be stated, such as: "Science fiction must involve itself with science and technology at least tangentially. It must deal with a society noticeably different from the real one of its time, and this difference must involve some change in the level of science and technology (...)" (Asimov, "Science Fiction Finds Its Voice")
Therefore science fiction has to discuss the future in respect of technological and scientific change as well. Kropf explains that as SF stories provide in their settings some sort of extrapolation of science and technology and that as this altered environment conditions the terms of a conflict, it will always provide an outlook onto the future towards which we are heading. (cf. Carl R. Kropf, "Douglas Adams's 'Hitchhiker' Novels", p.4)
The interesting combination of the two words "science" and "fiction" which might be interpreted wrongly as paradoxical is explained best by a citation of Brian W. Aldiss' "Introducing SF": "The best science fiction does not depend for its effect upon a slavish adherence to scientific accuracy; only some of the worst concerns itself with trying to achieve such adherence. The chief ingredient of SF - to use its more familiar name - is something that in literature has proved more vitalizing." (Aldiss, "Introducing SF")
He continues that a fair rough summary of SF might be that imagination prevails over skill and what is told is rather what one wishes than what one knows. (cf. Aldiss, "Introducing SF")
And as SF is such an unorthodox form of literature, in which you can find almost anything from new ideas which may be useless or real problems in future times to archetypes or paradigms which show up in the genre with ashaming regularity, Aldiss declares already in 1964 that "science fiction, more than any other form of writing, is typically the fiction of today.
"This is why an SF story can give you such a powerful stimulus: it is about what is happening to you." (Aldiss, "Introducing SF")
Why science fiction is the fiction of today may be explained by the interesting aspect of its variety. The common classification of fiction into different genres like love stories, criminal stories and adventure stories is not the correct stage to put SF stories on, because SF as a whole can include different types of stories as SF is the entire section of literature which deals with science and technology. And there are many love or detective stories which simply become SF, because the protagonists own a robot or a spaceship. "Von diesem Standpunkt aus betrachtet, ist die Science Fiction keine Spielart des Genres >Fiktion<, sondern eine weit gefächerte Gattung, die ihrerseits viele Genres umfaßt.
"(...) Es ist unbestritten, daß wir in einer sich schnell wandelnden Welt leben, in der das Heute veraltet ist, bevor die Sonne noch ganz untergegangen ist. (...) Die Science Fiction befaßt sich ihrer Natur nach mit Veränderungen. Sie akzeptiert die Notwendigkeit und die Unvermeidlichkeit von Veränderungen als Grundmerkmal ihres eigenen Wesens (...) Science Fiction behandelt die Konsequenzen der Veränderungen (...) Sie ist, kurz gesagt, die einzige Literaturgattung, die exakt auf die heutigen Probleme, Ängste und Hoffnungen eingestimmt ist - die einzig wirklich aktuelle Literaturgattung (...)" (Asimov, "Der Horizont der Science Fiction")
"From this point of view Science Fiction is not a kind of fiction, but a
wide-spread genre that includes many genres in itself.
"Undoubtedly we live in a fast-paced world in which the Today is obsolete before sunset. (...) Science Fiction quite naturally deals with change. It accepts the necessity and inevitability of change as a basic characteristic of its own essence (...) SF discusses the consequences of change (...) In brief, it is the only genre that is directed exactly towards today's problems, anxieties and hopes - the only really up-to-date genre (...)" (Asimov, "The horizon of Science Fiction")
"And this is why ordinary people in stories of the future seem so meaningful, so capture our imagination: they are us. They are us, magnified into what we hope or dread to be. Their moral predicaments and adventures reflect the predicaments and adventures of our inner lives." (Aldiss, "Introducing SF")
This thought leads to a very important feature of SF stories which many SF writers quite naturally stick to in order to sustain the reader's interest: closure. The endings of the stories must be logically deducible and have to satisfy the pattern-seeking tendency of the humand mind. "The reader wants to know 'how it all turned out' and in the light of the fictive conclusion to understand how all of the events and characters are ultimately related." (cf. Carl R. Kropf, "Douglas Adams's 'Hitchhiker' Novels", p.3)
But as this is a feature which is widely accepted for any kind of fiction I do not want to explain at length how it influences SF in general, I merely will refer to this fact later.
Concentrating on the essence of these introductory remarks and leaving out a full overview of the development of science fiction in literature in general and in English literature in specific, I will now investigate how much Douglas Adams fits into the criteria of SF as they were established by the writers cited above.