Q. This is the term used in Aviation Project Management to signify that the aircraft is ready to be used commercially.
A. EIS or Entry Into Service in Aviation Project Management signifies the end of the development process, when the aircraft is ready for delivery to customers, and induction into commercial service. For an aircraft to reach this stage, there are multiple hoops to jump through, and it generally involves a dedicated PMO working hand-in-hand with multiple internal development teams and vendors - from avionics, to engines and fuselage, and even training design and simulation. EIS also depends on the National Aviation regulatory body approving the aircraft and the training, as well as certifying the operating airline if it's a commercial operation. Professional Entry-Into-Service Management helps stakeholders to reach their goals.
[Source - Internet]