Software Engineering : The application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software; that is, the application of engineering to software. The study of approaches as stated above.
SE in Agile approaches has changed to a set of Core, Values, Principles and Practices that when followed will deliver Quality Software and that meets the needs of the Users.
For More on Agile approaches Read Agile Processes Page.
Software Engineering : A Practitioner's Approach by Roger Pressman.
A Discipline for Software Engineering by Watts Humphrey.