As a Word:-
There is no actual official documentation where word `engineering` came from but most of people believe its came from the latin word `Ingenium` that means `cleverness` and `Ingeinare` , means "To design or devise". Some also state that its came from word `Engine` means something that transform one form of engergy to one or other form energy that make job/task easier.
As The Concept:-
Engineering as a concept is just to do whatever it takes to solve problems that could be considered resistance in the way of Progress/Advancement of Human Civilization and Environment. Now here obviously problems are not something that bothering any specific individual but something that bothering to `mass of human` or group of humans for their day to day life. Or something that in some specific condition happened in past and chances to happened for future as well, that should not be happened so prevention of problem as well.
So Generalized definition of engineering is quest of solving and/or preventing problems by using science,technology,logic and mathematics or even metaphysics as a tool. and so engineer is the one who do engineering as a part of fulfilling his duties.
As The Mindset:-
To become a good engineer a person should have mindset that every problem has been raised due to occurrence some unwanted phenomena that happened as a result of specific situation and/or specific conditions of surrounding environment And this all phenomena occurrence follows specific equation or series of specific equations that have some factors as a constants (hard to change or alter) and some variables (easy to change/alter) and by changing these variables we could control and manipulate the phenomena so problem could be solved or prevented by `invention/creation of any tools/devices/machines` or `defining process with the help of tools/devices/machines`. So an engineer should not think that phenomena has been occurred as a bad part of bad luck or any kind of irrational reasons.
As a Job:-
To simpplify this duties of an Engineer described as problems that are complex and massive and one person could not master all of them. four core categories or i should write faculties has been derived and thats are Civil, Chemical, Electrical and Mechanical. and others has been derived from them as a result of some problems of each of them becomes more complex and demanded to solve by hybriding specialized knowledge and skill from multiple faculties of this core engineering fields.
Here are the most of fields of engineering classified as per its root fields.