Definition of Plagiarism

Plagiarism is :

• the practice of taking someone else’s work or ideas and passing them off as one’s own (Oxford Dictionaries)
• an act or instance of using or closely imitating the language and thoughts of another author without authorization and the representation of that author’s work as one’s own, by not crediting the original author;
• a piece of writing or other work reflecting such unauthorized use or imitation (Dictionary.com)

Synonyms : appropriation, infringement, piracy, counterfeiting ; theft, borrowing, cribbing, passing off (Dictionary.com) 

In short, plagiarism is considered by the UNamur authorities and your colleagues as « cheating » and/or a « fraud », whether voluntary or not (ignorance of the rules of citation in your area of studies/research, carelessness or naïve behaviour, etc).

Sanctions may not be the same depending on the degree of « malice » of the plagiarist, but the fact of plagiarism is serious whatever the intent!