The IEEE defines software engineering in the following way:
(1) The application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software; that is, the application of engineering to software. Source: http://www.idi.ntnu.no/grupper/su/publ/ese/ieee-se-glossary-610.12-1990.pdf
But what does systematic, disciplined and quantifiable mean in this context? Is there any further explanation from the IEEE?
You can rely on a dictionary:
http://dictionary.reference.com/browse/systematic: having, showing, or involving a system, method, or plan
http://dictionary.reference.com/browse/disciplined: having or exhibiting discipline; rigorous
http://dictionary.reference.com/browse/quantifiable: to determine, indicate, or express the quantity of.
So, applying software engineering requires the exact execution of method that is precise with quantified steps and exact procedures regarding development, operation and maintenance of software.
or as it is state at the end:
http://dictionary.reference.com/browse/engineering: the art or science of making practical application of the knowledge of sciences, as computer science, as in the construction of software. (I changed a little bit this definition) =)