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.
