An academic source is one written by and often reviewed closely by scholars with an academic expertise in the field they are writing and working in. These sources almost always use established ways of knowing and methodologies (research methods) that are widely accepted in the field and build upon past research and/or established knowledge in unique ways. Because of this, academic works tend to be much more specific than other types of sources.
The easiest way to tell if a source is academic is to google who wrote it and where it was published.
Peer Review in this context, means a specific process beyond editing that these sources go through. These sources are carefully read and evaluated by other academic experts in the same/a similar field. They look closely at the originality of the research question, the methods, the results or arguments, and how the author uses different types of evidence to support their claims. Most published academic works go through several rounds of review and revision before they are published.
Some signs you have an academic article:
Some signs you have an academic book: