One of my bugbears. I can only find one reference to it in EASA documents, and that is a learning objective in the ATPL syllabus, and no definition provided. There is a reference to service ceiling in one old FAA doc., the airplane flying handbook from memory, and it turns up in one FAA regulation, something to do with oxygen, or two pilots (or maybe even autopilot). I suspect the definitions in Wiki have been written to match the answers in EASA exam questions.