The American Engineers' Council for Professional Development (ECPD, the predecessor of ABET) has defined "engineering" as: The creative application of scientific principles to design or develop structures, machines, apparatus, or manufacturing processes, or works utilizing them singly or in combination; or to construct or operate the same with full cognizance of their design; or to forecast their behavior under specific operating conditions; all as respects an intended function, economics of operation or safety to life and property.
Software engineers take scientific methods created/researched/etc from Computer Scientists and created something from it. The difference between an engineer and joe shmo is that an engineer understands the science behind what they are doing.
An engineer could build a bridge and a regular guy could build a bridge. Both may work for 100 years, but the engineer KNOWS it will work for 100 years (under specific conditions obviously) and the regular guy doesn't. The same could be said for software.