What is the official definition of Multiservice Operational Test & Evaluation (MOT&E) and which reference can I find it in?
See Glossary of Defense Acquisition Acronyms and Terms at https://dap.dau.mil/glossary/pages/2274.aspx:
Multi-Service Test and Evaluation (T&E)
T&E conducted by two or more DoD components for systems to be acquired by more than one DoD component, or for a DoD component’s systems that have interfaces with equipment of another DoD component.
Numerous additional sets of information on the subject can be found with a simple web search using the term.