In "The Engineering Method" by Billy Vaughn Coen (1984), Coen states correctly that everything in engineering is heuristic.   A heuristic is something that helps the engineer make a design choice.  

The rule of engineering is: do what you think represents best practice at the time you must decide...the definition of the engineering method depends on the heuristic; the rule of the method and the rule of judgment are heuristics...all engineering is heuristic.   [Coen, 1984]

Coen lists some sample heuristics such as:

"Engineering is trial and error" or

"Work at the margin of solvable problems" or

"The yield strength of a material is equal to a 0.02 percent offset on the stress-strain curve"   

 This is what the engineer does, this is the engineering method.   Engineering is the use of engineering heuristics.   It is as simple (or complicated) as that.   My problem with this definition is only the word "heuristic", it is too complicated.    I think it should be called "design aids" or "engineering guides" to reveal a solution to an ill-defined problem (as opposed to well defined problems of applied science).    So in this sense, engineering is a decision making process by an engineer who applies know-how and personal values (similar to my definition).