Many people have offered definitions of engineering. You can find most of them via Google. A reasonable one is this:
“Engineering is the application of science and technology to meet identified needs.”
There's quite a lot built into this definition.
Science is generally the undertaking that seeks to understand real phenomena such that we can predict their behaviour. As a rule, science tries to keep at arms length - to not influence - the objects of study. Technology is “the study of or a collection of techniques; a particular technological concept; the body of tools and other implements produced by a given society” (from http://en.wiktionary.org/wiki/technology).
Engineering is different: it seeks to actively alter reality - while obeying the laws of nature - for specific purposes. So while science seeks to understand the universe, and technology is the sum of everything we have already learnt to do, engineering seeks to change the universe as it is now.
This means we are always aiming at a moving target: every new engineered artifact that is introduced into the world (i.e. a new element of technology), changes the world, which changes the way people live and the context in which we will engineer the next artifact.
Every artifact induces change, and those changes can often have very broad impacts. We can rarely predict the kinds and intensity of impacts that those changes will have. (Humans are generally very bad at predicting the future.) This means that any artifact could have unintended harmful consequences that are essentially unavoidable.
Thinking exercise: Name as many unintended harmful consequences of engineered artifacts as possible; make sure you can also identify the artifact causing the harmful impact.
The most obvious way to minimize the harmful but unavoidable consequences of engineered artifacts is to make new artifacts that are as similar as possible to existent artifacts that we know (or at least are very confident that) have very few, relatively minor harmful consequences. This is one of the reasons why engineers tend to be quite conservative, professionally.
Another way to minimize harmful consequences is to introduce new products very gradually, to a pre-defined user group, all the members of which are aware of the potential risks, willing to assume them, and (usually) have some agreement with the engineers to collaborate with them to report and solve problems that do emerge.
Engineering is also as broad as science. That is, there is no restriction on which sciences are fair game to be applied by engineers. Engineers tend to focus on the natural sciences, because when science was invented, engineering dealt predominantly with natural artifacts necessary for survival (principally shelter and weaponry), and the two became tightly connected a very long time ago. However, science has grown tremendously in the last five centuries, and so too have the possibilities for engineering. So, really, any science can be applied by way of engineering.
Thinking exercise: Think of as many sciences as you can. For each science, think of what artifacts could be engineered by its application.
Another piece of the definition of engineering explains why we do it: to meet identified needs.
These needs are the needs of humans. Since these human clients are rarely synonymous with the engineers themselves, engineering is inherently a service to others. Engineers are agents who act for their clients. (This is at the root of the most important ethical obligation of the engineer: to protect the public good. Note the similarity between the engineer's obligation to the public good and the Hypocratic Oath of medicine.)
There are many issues here:
Each engineer needs to discover his/her own answers are to these questions. They are at the root of one's personal dedication to the profession. We will see that elements of your answers to these questions arise from understanding about design.
In the end, we engineer to make life better for humanity. (This assumes that meeting one's needs is equivalent to making life better, but this is not always the case.) And while we may derive personal satisfaction and enjoyment from engineering, such benefits are not by themselves sufficient.
Exercise for the reader: Can you identify the things in the following list that are engineered artifacts?
Considering the items in the list that were engineered, explain why you think they were engineered. From this, try to enumerate some of the characteristics of engineering.
Engineering without design is like science without research.Tom Brzustowski, President of NSERC, 2004
Once upon a time, there were no engineers, and people made things by trial and error. If one needed something, one would just start making it. If it worked, which it rarely did, then that was that. If it didn't work, then one would try again, leveraging their experience of the first attempt to try something different. If one because particularly good at making something, one was known as an artisan or craftsperson, and was considered in very high regard. Craftspersons would spend significant time training younger apprentices because there was no codification of the knowledge of the craftspersons.
Engineering, which was established during the Industrial Revolution, is different, because engineers plan things before building them. We have to plan them because over time products have become more complex, and people have become less tolerant of any kind of product failure. The planning stage is where design happens. It lets engineers prepare to create ever more complex and powerful products, looking at ever finer detail of how a product works and how it will impact the situations in which it will be used.
So, at its heart, engineering consists of two main tasks: first we design a product, then we manufacture it. Of course, engineering is not that simple; but everything else about engineering emerges from these two essential tasks.
This leads us to ask: What is engineering design?