The first and foremost part of research is that it must be ethically conducted and with originality and novelty. Now the plagiarism is the scary word because it’s opposite to mentioned merit or characteristics of research.
In very common way its "copying" or "borrowing" or even you say "intellectual cheating". These word may seem little light in meaning but have consider as a fraud made by researcher.
Plagiarism character is intent to steal someone else idea and give it his or her own name to that idea. So basically plagiarism is a "theft of ideas or intellectual properties" for getting personal benefit or gaining. So, it's clear that plagiarism is act of fraudulent and great concern of academic dishonesty.
Now this academic dishonesty varies from universities/institutions, though there are clear directive available from regulatory bodies but the culture and knowledge of professors or faculties played important role in this concern.
We are aware about plagiarism and must be mentioning that not all students or researchers do it intentionally. But this is also a great matter of concern that because of inadequate knowledge or guidance almost every researcher end up with some or more plagiarized content. But unfortunately whether knowingly or unknowingly, this plagiarism leads to penalty.
There are various ways by which academic dishonesty or plagiarism may happen like
Plagiarism may cause lots of consequences like: ethical and professional (harming reputation, suspending from job or your research, invaliding research etc.), legal (almost all countries keep safe by forcing law on plagiarism and India also have strong law against plagiarism) and financial consequences.