I saw many questions which have been answered by copying whole sections from e.g. NOAD.
I wonder whether there is a limit regarding the comprehension.
I would argue that in most cases, as long as the citation isn't too extensive, that it falls under the Fair Use exception of US copyright law.
Excerpt from Cornell Law School page on the topic
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include-
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work. The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
I worry every time I quote a book in an answer about whether I’m being considerate to the authors.
I don’t know if an official policy is necessary or what it should be. In the absence of a policy, here’s what I wish people would do:
If you quote, quote accurately, use quotes or
> to mark quotations clearly, put your commentary outside the quotes, and of course always cite the original author.
If you summarize or paraphrase, cite the original author anyway.
Keep block quotes short. It’s unkind to make an author’s work, which is their livelihood, available to everyone for free against their wishes. And anyway the best answers are short.
If you like a book, recommend it and link to the author or publisher’s page.
This long block quote strikes me as too much. It looks like a whole main entry from a reference work.
from Wikipedia Copyrights describe this issue quite well.
"... it is legal to read an encyclopedia article or other work, reformulate the concepts in your own words, and submit it to Wikipedia, so long as you do not follow the source too closely"