If an internet user search some thing with word or word phrases at a search engine sites like Google, Yahoo, msn...then that word or word phrases will be the Keyword. On the other hand, The word or word phrases by which you like to visible your site in search engine sites is also called as Keyword.
In SEO industries keyword is main part of the websites you can get good traffic, business and good backlink through choose right keyword.
Keywords are words or phrases search on Google may it be correctly spelled or not. Its still a keyword and that is a keyword. get it?
I think every keyword has its own definition. Its depends upon you that which keyword you have to choose for definition.
A keyword may contain one word or more than one word, it tells main purpose/function of a product or service in SEO.
The user search's for specific query in the search engines. The specific query is a word or combination of words which is called keyword or keyword phrases.
Keyword is the the combination of the alphabets which user enters in search engine to begin the search.
You said keywords may or may not be correctly spelled. Only those which are spelled correctly are the keywords. Am I wrong here?