In many discussions involving rules interpretations the meaning of words is often called into question. Often, people will claim that if the game doesn't define the term that it defaults to the plain English reading of the word.
Is this stated somewhere explicitly in the rules or in designer comments? What is the source (or sources) for this claim?
Jeremy Crawford1 has affirmed that this is indeed the way the rules are supposed to be read in this tweet:
Unless the rules explicitly expand, narrow, or completely redefine a word, that word retains the meaning it has in idiomatic English. #DnD
Going back to the original articles detailing the design goals for the 5th edition (see this related answer for more details) one can also find that there was some concern over the kind of language used to detail the rules. Of particular interest is this article (found by illustro) which includes:
The choice between "fun to read" and "precise" needs to be handled on a case-by-case basis. Certain rules can be simple and straightforward, while other matters can be handled more conversationally or filled with inspirational descriptions of people, places, or events.
While certainly not as explicit as the tweet by Jeremy Crawford, one can read from that article that the designers wanted to use plain language to describe at least part of the rules where more specific technical jargon was not necessary.
1. Jeremy Crawford was the lead rules designer of Dungeons & Dragons 5e. Though at the time this answer was first written his twitter posts were considered official, as noted by V2Blast, that is no longer the case.
In addition to the tweet given in Sdjz's answer, those interested in a more long-form description of Crawford's perspective on this can listen to him on the "Sage Advice" portion of a 2017 DragonTalk podcast here. The context of the interview was what target and targeting mean, but as part of this he states the 'idiomatic English' principle (bold emphasis mine).
[10:30]
We should pause a moment and define our terms. What the heck do I even mean by 'a target'? Sometimes people will debate 'Well, what does 'target' mean?' Partly because 'targeting' was a closely-defined concept in the previous edition of the game, 4th edition, and so players who moved from that edition to 5th edition will often expect a similar precision in 5th about what is a target. In the spells and other powers of 4th edition, every one...
[11:00]
of them had a line called 'target'; the target of an effect was very carefully defined. In 5th edition, it's a bit more open-ended because the word 'target' doesn't actually have any game meaning in 5th edition. The rules use the word 'target' a lot, so the word is certainly very important to the rules, but at no point do the rules re-define what that word means, you know...
[11:30]
for the game, or give it any special meaning beyond what the word means in English. And really this is a general principle in our rules: if the rules do not specifically add meaning to a word or take meaning away, or completely change the meaning, it simply means what it means in idiomatic English. And...and...So when the rules say 'target', they really mean the English definition...
[12:00]
(and granted, many English words have multiple meanings, so it's understandable that sometimes there's confusion), but the meaning that the rules are getting at, is that when you choose someone or something to be subjected to some kind of effect. That's one of the common ways that the word 'target' is used in English.
Later on in the interview, which in total is about 35 minutes on what targeting means, Crawford talks about about another meaning of 'target' used when there is no active choice involved (such as the targets of a fireball spell).
[Note that the above is my own transcription from the audio recording, and I have left out some filler words from both Crawford and his interviewer]
