What makes a journal scientific? Question
What exactly is the definition of a scientific publication or journal?

Background
What fueled this question in particular is the current rise of predatory publishers. Jeffrey Beall had a list published online of suspected predatory publishers, but his blog has very recently been discontinued (cached copy, 11 Jan 2017). Speculations of this disappearance include threats and political motivations. 

Anyway, predatory publishers are characterized by any or all of the following:


They appear suddenly and generate a host of new journals that cover a wide range of topics;
They are open access and charge high fees;
They have rudimentary, faked, or even no review process at all;
They spam about every researcher with a publication record asking them to publish in their journals;
They spam researchers to join their board of editors or become a reviewer.  


For more backgrounds on predatory publishers see the wiki page.

Now still these predatory publishers profile themselves as being scientific publishers, e.g., Scientific Research and Austin Publishing Group.
 Access these links at your own risk. I would not call this class of journals scientific, as pretty much any author willing to pay a few grand can make it in there. 

So what makes a scientific journal scientific? How exactly is this defined? I long thought a peer review process guarantees scientificness, but it can be faked or even absent, yet publishers claim to be scientific. 

Where this question becomes particularly relevant is in the role of teaching - when I tell students only to use 'scientific literature' then what is it exactly? Would it be more a matter of what it is not?
There is no general definition for scientific journal. The Wikipedia definition is as vague as reality is. This is because there is no clear definition of scientific. It is a case-by-case distinction - not only for journals, but also for individual articles in a journal or one method described in an article. Additionally, scientific and predatory are not mutually exclusive. 

Your instruction to your students to only use scientific literature is the key to the answer to your question: What is scientific and what is not depends on the concrete content of an article or the applied method. To distinguish between these two is the first step towards scientific thinking.

An answer on a list of predatory publishers can be found in this question here.