Citing a definition is not plagiarism. Of course you are allowed to quote individual sentences from others or include equations and definitions which naturally must be identical in several papers...

Plagiarism means that you falsely claim to be the original author of a work. When you quote others, you should clearly show that you are quoting, by using quotation marks, italicizing or the like. Better yet, write it out explicitly :"This is a quote from... " Also, cite the original source where the definition was first stated (not any other who have also quoted it) and give due credit to the people who originally wrote it, perhaps by mentioning them by name.

thanq sir for the answer....@Jonas Nyman

I agree with Mr/Dr Nyman. Here is an example I am making up as illustration:

On the matter of plagiarism, of concern to many researchers, I would agree with Nyman (2018) who asserts in a posting on Researchgate that "Plagiarism means that you falsely claim to be the original author of the work. When you quote others...or the like."

I need to add that although something appears on Researchgate, it may be modified or even deleted -- no offence meant Mr/Dr Nyman -- I am trying to make the point that postings on Researchgate are NOT permanent and can disappear.

Reference: J Nyman, personal communication, February 22, 2018 in Researchgate.

I suggest you refer to APA guidelines or Harvard Referencing for a definitive answer. Perhaps both have nothing on this as posting are not immutable and not permanent.

I hope this helps. Or, just quote WHO in this manner: WHO says " a b a c".

thanq sir...