Here is a word cloud created with wordle.net . The input was the definition taken from ISO/IEC/IEEE 24765, which combines a slightly modified version of the definitions of IEEE 610.12 and the definition given in the PMBOK 4th edition. I will add new versions of the word cloud once new (citable) definitions come in. [Unfortunately I did neither find a way to update my question with new attachments nor to update this answer with a modified attachment. I asked RG team for advice, but no answer yet.]

Here is a different word cloud, this time incorporating definitions from these three standards:

[1] “ISO/IEC/IEEE 24765:2010 - Systems and software engineering -- Vocabulary,” ISO/IEC/IEEE, 24765, Dec. 2010.

[2] “ISO/IEC 26702 IEEE Std 1220-2005 - Systems engineering — Application and management of the systems engineering process,” ISO/IEC/IEEE, Jul. 2007.

[3] C. Haskins, K. Forsberg, M. Krueger, D. Walden, and R. D. Hamelin, Eds., Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities. Version 3.2.1. San Diego, CA, USA: International Council on Systems Engineering (INCOSE), 2011, pp. 1–384.

Have you reviewed some "classic books" in Software Engineering from authors like Sommerville or Pressman?

I think the problem you get with definitions on words like this is the ambiguity in use and the subjectiveness in how the terms are framed within each culture and environment. How one company, or culture, uses the term is not always the same as another and hidden meanings or subtexts often change the definitions and usage. I've also seen this across departments who have differing philosophies on how projects are scoped or defined such that one group can look at a requirement as hard and fast and once decided upon it should be implemented while another may see it as fluid.

@Samuel: Yes I did that, but the opinions what is "classic" differ. :-) Regarding the two authors you mentioned, which of the two definitions do you like best?

@Michael: You are right, but I was asking myself whether there was a definition that many companies and individuals prefer over others. I did not want to start a hot discussion what *is* the best definition, but rather collect the opinions of RE experts and then assemble a word cloud from it.

Florian, in my experience definitions on certain terms become overloaded with history and politics, while many words are common ground across companies and departments you will end up with variances locally. Although when you ask whether some people have a favorite definition it usually ends up in a discussion as people refer to standards or expert definitions.

To get your question though, when I use the work requirement I take it to mean some specific point of functionality or behavior that is intended to happen, occur or be generated by some specific action. I am general in that I have seen requirements go from wide ranging to exacting depending on the level of documentation and the project.

I like the swebook definition, a requirement is a property which must be exhibited in order to solve some problem in the real world. 

Michael and Jose, I added your choices to the word cloud from above. Hope to see more votes :-)

You may read the followings for a better understanding of the requirements in context with current trends:

Poster Towards a Requirements Engineering Artefact Model in the Con...

https://ieeexplore.ieee.org/document/8501349

Conference Paper Understanding quality requirements in the context of big data systems

Conference Paper A Validation Study of a Requirements Engineering Artefact Mo...

Conference Paper An Approach for Deriving, Analyzing, and Organizing Requirem...

Conference Paper Deriving Metrics for Estimating the Effort Needed in Require...

Article Requirements Engineering in the Context of Big Data Applications

Chapter State of Requirements Engineering Research in the Context of...

Conference Paper Towards a theory on the interaction between requirements eng...

Some of these papers have no full-text availability, if you need, please knock me.

Dear Florian Schneider,

In my Systems & Requirements Engineering discipline I use following definition for Requirements:

"A requirement is an affirmation of a need, something that a user or any other interested party (stakeholder) requires in a given system or project."

The IEEE has the following definition for Requirements:

"The ability or attribute that must be met or exist in a system or component of the system to satisfy a formally or legally applicable contract, specification, standard or other document."

Regards,

Prof. Wiltgen