In short, the answer is "yes". It is called "self plagiarism".  However, one can cite his/her own work. Some people think self citation is good as it shows your expertise in the field. I don't self cite unless it is necessary.

Dear Chaudhary, Very relevant question. I appreciate it.

Dear Kisalya Basu sir,Namaskar and Shubho Vijaya Dashami.

Many authors wonder if it were plagiarism copying a portion from their own works. I believe it is not something we might call plagiarism (let alone self-plagiarism), as long as the author holds copyrights and related rights on the work; and the work is properly cited. So much of latitude and flexibility on part of the authors/ researchers should be permitted if we were to promote research and writing. I agree with you that some believe self citation is good as it shows your expertise in the field. Congrats. By the way, may I bring to your kind notice that I'm the younger brother of your co-author Dr. Sameer Sthapit (Rajbhandary)? All the best, sir

Yes, it is. Any portion, however small, used without acknowledgement is plagiarism. Authors should have some humility in acknowledging someone else's work, one is not humiliated if (s)he acknowledges others..

Agreed Saikat Sinha Roy

I don't know the legal side of it, but  I agree that any portion taken from anywhere should be acknowledged.

the terms for this are "extended version" "parts of this were already published as", and so on. There are tons of articles and chapters based on formerly published work. This is your own work, after all. You might call it "synergy", but defenitely not plagiarism ;o)

Text you publish in a journal article should not be re-used without providing a reference.

However, you can re-use the IDEAS and simply re-write the sentences to say the same thing in a different way.  

Plagiarism is a serious offense in academics. One should think twice before alleging a researcher to be a plagiarist. From the legal point of view, ‘plagiarism’ means copying from others’ works. I’m of the opinion that if one copies from his own earlier works without citing the sources cannot in many cases be called a plagiarist. I mainly perform policy-based theoretical research on trade and development. I have introduced a few concepts which are not well-known in the literature. In many cases, while using those concepts in a new work, I had been asked by the referees to explain them. I know even if I mention the sources, an average referee would not waste his time to see the relevant portion(s) from the cited works. In such a case, if I copy from my earlier papers to define the concepts in a few footnotes I don’t find any reason why I should cite the sources or rephrase words because these have already been defined earlier. Even if I do, the referees might think that I claim to have done some decent jobs earlier which I think I haven’t. Besides, in some cases, I am asked to provide some empirical evidence in support of certain statements that I have already provided in some earlier papers. Furthermore, in the type of research that I mostly perform, I require describing an economy that I’m considering. In such a type of works, certain portions including mathematical symbols, assumptions etc. must have to be the same. The point is that by simply copying those portions from earlier works, I’m not contributing anything new to the existing knowledge because these are very minor parts of the new work. Hence, I do find no reason in wasting time in rewriting those insignificant portions. However, if I replicate the idea of a published work in a new paper, I do agree I’m a plagiarist.

I agree with Prof. Sarbajit Choudhury, but not fully. What's the harm if I cite my previous work properly while using that concept even in the footnotes? spending that little-bit of time actually enhances my credit as well as credibility.