The basic definition of "turnkey" itself isoted in visual imagery; just picture getting a key to a car, inserting the key, and having it start without any problems. If a person makes use of the term "turnkey", what it is they are reasonably mentioning is a thing which is made for instant using (this term is mostly used for the sale or when one offers the supply of services and / or goods) . Common examples include a "turnkey" home which is completely built and ready to be lived in (having everything from the construction to the carpeting and cabinets being finished), or another being a "turnkey" business where 3rd parties contracted are used to set up certain locations or needed equipment (ie the rail lines or a train) and putting them into operation, after which they turn it over to the one who hired them.
Using that line of reasoning, the definition of a turnkey eCommerce site would be when a brand new eCommerce site is completely created by a hired entity who will then turn the finished website over to the owner. In order for a website to be able to claim to be a real turnkey eCommerce site, it must have, from when it is turned over to the owner, a usable backend (the administration site) allowing important changes and updates (ie products, pictures , text, categories, subcategories, etc) to show up on the front end (the site a customer sees). In essence, the owner must have total control of the site from the get go in order to be defined as a turnkey eCommerce site.
So, for example, say have get the idea for creating a eCommerce site that imitates a more famous eCommerce website such as Amazon.com (not stealing the site's layout or format naturally!). In this case, you could locate a turnkey eCommerce company, tell them as to what your needs are (with some helpful examples), have them set up the site to your specifications, pay them, and receive the proverbial keys for your set up eCommerce site that can sell your own niche products. You can even have them help you to set up a url and a domain hosting service for a small fee (when it is not included) if you need it.
Being as the eCommerce field is a savagely competitive one, it probably is not smart if you go for an open or closed source do-it-yourself eCommerce website design, even if it is said to be simple to set up. Finding and hiring a capable maker of turnkey sites is the easiest and fastest way for those interested in getting into eCommerce.