Is verbatim copying of short technical phrases without quotes but with citation considered plagiarism? My question is similar to Is verbatim copying several paragraphs of text with citation considered plagiarism?, but
I want to ask about very short, concise wordings. As an example, I read a paper in which the author uses the following sentence:


  While in the above description we have specified a local algorithm as
  a function that maps local neighbourhoods to local outputs, we could
  equally well […]


Now, instead of copying the exact sentence with its grammatical structure, I
want to point the reader to this paper and briefly mention one of the
conclusions made in this paper. Consider the sentence


  author et al. [citation] show that a local algorithm is a function that maps local neighborhoods to local outputs.


The wording "function that maps local neighborhoods to local
outputs" is a verbatim copy from the original source. Of course, I could
replace this wording with something else that expresses the same, but I find the original citation very concise and I could not come up with a completely
different and equally concise sentence.

I always use quotes in addition to a citation to tell the reader that not only
the conveyed ideas and concepts, but also the wording, is not my own
intellectual achievement. But in this case, one might argue that a person that
has understood the ideas described in the original paper might come up with the exact same wording, hence the wording is not a result of the original author's linguistic style, but rather a direct conclusion of the idea he wants to explain.

Is it acceptable to use this wording with a citation and no quotations, or
should you always use quotes for verbatim-copies?
I disagree with the two existing answers (gerrit and Patric).

In mathematical writing, it is not necessary to put quotation marks around very short fragments of descriptive text where that text is the obvious and natural way to express the idea.  For example, if Smith has written a paper whose main result is


  Theorem. Every even number is divisible by two.


then it is perfectly acceptable to write


  Smith [cite] shows that every even number is divisible by two.


without quotation marks and without clumsy rephrasings such as


  Smith [cite] shows that all even numbers have two as a factor.


The significant intellectual contribution of the work you're citing is the theorem itself, not the obvious wording that they used to express it. As you say, anybody who understood the concept would probably choose to phrase it in that way, even if they'd never seen the paper you're citing. Mathematical writing would be completely unreadable if every phrase that had ever appeared before was put in quotation marks.  After all, Smith was hardly the first author to talk about even numbers&nbsp;&ndash; are we going to accuse him of plagiarism for not acknowledging that the phrase "even number" is a quotation from somebody else?