Requirement Engineering and SDLC

What is Requirements Engineering? Does it only include pre-development phases of SDLC only? What is its purpose?

Requirement Engineering is the most important phase of software development process. During this process, all the requirements for the proposed software system are discovered, gathered and analyzed. The most basic purpose of requirement engineering process is to establish the goals, functions and constraints of the software system to be developed. If proper attention is not given to this process, then the developed system may lack some of the required features or functionalities and as a result system may not satisfy the needs of some users and hence may fail.