I found this term on the homepage of a research lab ... (http://www.csirmadrascomplex.gov.in/nml.html)
When you bu a car it is a finished product such that the only thing you have to do to operate it is turn the key. A new home built by a contractor may also be considered a turn key since the new owner would not have to do anything but turn the key on the front door and move in.
Turn key then is a term used to indicate that nothing else needs to be done in order to use the product.
When used in an engineering sense it is similar to the above examples in that all of the design work has been done and may even include a finished product.
I contract an engineering company to design and build a new factory for me. After giving them some specifications and a pile of money I get a new factory ready to run and produce something. I turn the key to open the door to the factory and I can start using the facilities without having to do anymore building, fabrication and/or designing.
You can apply this term to smaller systems or large factories.
Your link didn't work for me, so I'm not certain if they're referring to the standard definition or not...
Typically, "turnkey engineering" means they'll provide a comprehensive engineering package that you'll just have to get in and "turn the key". It will include everything from the economic evaluation up through some final stage. The stopping point can be anything from a completed engineering package, just on paper, to including necessary permitting, to completion of construction, to completion of start-up and personnel training. It depends on the agreement and the nature of the project. At a minimum, the completed engineering design ON PAPER will be provided.
The term turnkey is also often used in the technology industry, most commonly to describe pre-built computer "packages" in which everything needed to perform a certain type of task (e.g. audio editing) is put together by the supplier and sold as a bundle. This often includes a computer with pre-installed software, various types of hardware, and accessories other A turnkey or a turnkey project is a project in which separate entities are responsible for setting up a plant or equipment (e.g. trains/infrastructure) and for putting it into operation. It can include contractual actions at least through the system, subsystem, or equipment installation phase and may include follow-on contractual actions, such as testing, training, logistical, and operational support. It is often given to the best bidder in a procurement process. Turnkey projects can also be extended, known as turnkey plus, where there is perhaps a small equity interest by the supplier and it will later on continue its operation through a management contract or licensing. Turnkey refers to something that is ready for immediate use, generally used in the sale or supply of goods or services. The term is common in the construction industry, for instance, in which it refers to the bundling of materials and labor by sub-contractors. A "turnkey" job by a plumber would include the parts (toilets, tub, faucets, pipes, etc.) as well as the plumber's labor, without any contribution by the general contractors. This is commonly used in motorsports to describe a car being sold with drivetrain (engine, transmission, etc.). A vehicle for sale without these pieces is sold "rolling". A racer may prefer to keep the drivetrain pieces to use in another vehicle to preserve a combination. Similarly, this term may be used to advertise the sale of an established business, including all the equipment necessary to run it, or by a B2B supplier providing complete packages for business start-up.
It is when you hand the whole contract on to a single contractor.