Opinions differ when it comes to the question asked in the title. For some, a text has to cite other authors to make it academic. For others, it needs to be factual – that is without personal statements. For still others, a text has to sound complicated and full of jargon. We might find yet other views. None of them are completely wrong, but each only highlights one aspect of a bigger picture.
A text becomes academic, when it fulfills (a minimum of) the following conditions:
The text connects to other texts and contributes new insights to a discussion. The contribution may be intended for a special disciplinary community or a wider audience of scholars. In any case, the addressed community decides whether the text contributes to a discussion and, therefore, forms part of an academic conversation.
To contribute, authors need to draw on the insights of other scholars by referring to them in paraphrases, quotes, and citations. The authors should evaluate knowledge to see where and how they can contribute new insights (finding the niche, as John Swales puts it). When they publish a text, other authors may critically reevaluate the research to provide new insights themselves.
The text presents the knowledge and insights in a factual way, leaving out personal opinions and views. Authors justify their questions, theses, and arguments; they define concepts and explain methods. Their readers should be able to understand and replicate the research.
The authors use language that is as clear and simple as possible. Using complicated sentences and jargon for its own sake goes against the goal of academic texts: to communicate insights (see the various style guides and manuals). Scholars can still use technical terms and elaborate concepts, but they should use them consciously and precisely. This aspect refers again to the second point above: other scholars should be able to understand and evaluate a text. If they cannot understand an argument because of imprecise or cluttered language, it is the authors’ fault, not the readers’.
If a text fulfills these conditions, an academic conversation can continue. Depending on the academic community, one or another of these aspects (or further ones) become more important.
If your texts fulfill these conditions, you should be on the safe side. Depending on the context and its academic requirements, you must adapt. As with everything else when writing, make conscious decisions that you can justify – whether they diverge from demands and conventions or stick to them.
PS: Do the famous texts by scholars such as Foucault, Bourdieu, Butler, Luhmann, and others represent texts in the above sense? I’m not so sure that that is appropriate in every case, although they contribute new insights.