The question at first seems so basic and laughably naive as not even to warrant a response. An English word, of course, is something you find in the dictionary. It is a group of letters put together with an agreed-upon pronunciation (unless you are from the British Commonwealth of Nations) that communicate clear meaning. But when you spend a little time thinking about it, you really don't know the answer. The Oxford English Dictionary (OED) lists more than a dozen definitions of "word," many of which have multiple senses. The one we are most familiar with is definition 12: "Any of the sequences of one or more sounds or morphemes (intuitively recognized by native speakers as) constituting the basic units of meaningful speech used in forming a sentence or utterance in language. . ."
Even if we become clear on our basic understanding of a word (and meaning is increasingly communicated these days in two or three-word phrases, rather than individual morphemes), we then have the problem of what an English word is. When does a word which originated "elsewhere" but is used frequently "here" become an English word? For example, as I was putting together my list, I realized that many of the foods I liked to eat were in few English language dictionaries; many of the stories I read in Spanish, for example, were called patasolas or sayonas, words appearing in no English dictionary. Well, then I decided to sit down to a dinner of Lachsschinken (should the "L" be capitalized?) and that word also doesn't appear in the OED, though it is in Webster's. It made me wonder for a second whether my food was real or half-real.
To make matters worse, when we can agree on which words make it into our dictionaries (and comparing two dictionaries with each other will show vast differences in what constitutes an English word), we often can't agree on how to spell them. For example, when I was growing up the Muslim holy book was the Koran. Now it is the Quran. When you look up words in the OED, you see that they often were spelled in sometimes half a dozen different ways over their history. Certainly there is a committee somewhere that would like to decide how to spell words taken from other languages as well as those originating in our own English-speaking cultures, but it all becomes a matter of convention and convenience.
Spelling bees are often the means by which people can compete with each other in their knowledge of English words but one has to read the fine print of the bees--each of them depends on the spelling in a particular dictionary to determine what is a correct or incorrect spelling.
In compiling the list that follows, a list that I put together from June 2017-January 2020 with occasional additions as I keep reading, I decided to draw upon both the standard American unabridged dictionary (Webster's Third New International) as well as the OED. But I often supplemented these dictionaries by words from countless web sites dedicated to a particular item, such as the names of religious garments and practices in the Greek Orthodox Church or Turkish foods that are popular in America or Sumo Wrestling terminology. In addition, I haven't aimed for the OED's ideal of comprehensiveness in presentation of words. Though I often give many words that come from the same root, I don't, for example, list (like the OED): "terrific, terrifical, terrifically." "Terrific" is enough for me. I give "terrifying and terrify" but don't list the OED words "terrifier, terrified, terrifyingness." I try to invite you into the family or basic meaning of a word and not the complete list of ways that it or its relatives have been used.
This list remains incomplete, of course, and it perhaps contains errors. If you find some, please contact me so I can make it right. Finally, I am now in the process of compiling short (150-200 word) definitions of many of these words. I hope to be able to give them on this site, perhaps about 100 words at a time, over the next few years. Thus, the "naked" words must ultimately be put into their "dress" so that they can look their best.
My one book to date on words is my 2009 book Word Wealth: 300 Words for Pleasure and Profit (Create Space). My previous website had nearly 1000 essays on words, many of them written in preparation for my participation in the annual Senior Spelling Bee, in which I participated from 2004-2011. I will resurrect a few of those on this site, but most of the material here is new.