Some definitions
Every good dictionary (e.g. The Oxford Dictionary of English) defines design as both a noun and a verb. The noun refers to a plan or drawing intended to show the look and function or workings of an artefact, while the verb usually means ‘to do’, ‘to plan’ or ‘to decide’ on the look and functioning of this artefact (i.e. ‘the design’).
The ambiguous nature of the word is illustrated by a seemingly nonsensical sentence:
‘Design is to design a design to produce a design’ (Heskett, 2002, p.3)
Design is a ubiquitous word; we see it often and in many different contexts: schools of design, designer clothes, design prizes, design shops, and everywhere we look we see design and designs. Some theorist even claim that ‘[a]ll what we [humans] do, almost all the time, is design, for design is basic to all human activity. The planning and patterning of any act toward a desired, foreseeable end constitutes the design process’ (Papanek, 1985, p.3).
It seems fair to reckon that since everyone seems to be doing it, then everyone should know what design is, and therefore what designers do and how they do it – or at least have a vague idea – but this is hardly the case. The troublesome definition of design remains elusive. Lawson (2006, p.33) asks rhetorically:
Do we really need a simple definition of design or should we accept that design is too complex a matter to be summarised in less than a book? The answer is probably that we shall never really find a single satisfactory definition but that the searching is probably much more important than the finding.
Even in a canonical source such as the aforementioned one, the reader has to wait until page 31 for an attempt to a definition of what design is and is not. After citing and reflecting on definitions by other authors, Lawson (2006, p.33) finally refers to Jones (1970) in a phrase that Jones himself regarded as the ‘ultimate definition’ of design: ‘to initiate change in man-made things’. This definition is not instrumental at all, but every designer should probably agree, at least intuitively and to some extent, that it resembles to what we do.
Nigel Cross does not provide a definitive definition of design in Designerly Ways of Knowing (Cross, 2007a) – his compilation of canonical papers – either, but instead he offers a fragmented description of characteristics that can be put together as follows: ‘design is rhetorical’ (p.51), ‘design is exploratory’ (p.52), ‘design is emergent’ (p.52), ‘design is abductive’ (p.53), ‘design is reflective’ (p.53), ‘design is ambiguous’ (p.54), and ‘design is risky’ (p.54).
A definition always focuses on certain aspects of the object the author wishes to explain. Often this focus lies in the process or planning of change such as in the aforementioned definitions by Papanek or Jones. The British Design Council also puts forward a definition by Sir George Cox that involves a final goal: ‘Design […] shapes ideas to become practical and attractive propositions for users or customers. Design may be described as creativity deployed to a specific end’ (Design Council, year n/a).
Simon (1996, p.111) poses a very famous and short definition that combines the view of design as a common human activity with a goal orientation: ‘every one designs who devises courses of action aimed at changing existing situations into preferred ones’. Simon (1996, p.114) takes his teleological view further and adds the artefact as a means to achieving goals: ‘[d]esign […] is concerned with how things ought to be, with devising artifacts to attain goals’. Other authors, point on the final results of the activities without focusing on end goals: ‘the process of creating tangible artifacts to meet intangible human needs’ (Moran and Carroll cited in Visser, 2006, p.115). Visser (2006) mentions other definitions that go in this direction and neglect to mention the crucial difference between the specification and the artefact product itself. This difference is what separates design from craft and it is at the very basis of design methodology (Jones, 1992).
The following all-encompassing definition (Visser, 2006, p.116) provides an accurate image of design:
Design consists in specifying an artifact (the artifact product), given requirements that indicate – generally, neither explicitly, nor completely – one or more functions to be fulfilled, and needs and goals to be satisfied by the artifact under certain conditions (expressed by constraints). At a cognitive level, this specification activity consists of constructing (generating, transforming, and evaluating) representations of the artifact until they are so precise, concrete and detailed that the resulting representations – ‘the specifications’ – specify explicitly and completely the implementation of the artifact product. This construction is iterative: many intermediate representations are generated, transformed and evaluated prior to delivery of the specifications that constitute the final design representation of the artifact product together with its implementation. The difference between the final and the intermediate artifacts (representations) is a question of degree of specification, completeness and abstraction (concretization and precision).
So what is design?
Design is the generative, transformative and evaluative activity behind these artefacts:

Sketches (source unknown)
And behind these:
And these:
And this too is design:
So then, what is design?
As John Chris Jones argues, design is, above all things, ‘the performing of a very complicated act of faith’ (cited in Lawson, 2006).
References
CROSS, N. (2007) Designerly Ways of Knowing, Basel: Birkhäuser.
DESIGN COUNCIL (year n/a) What design is and why it matters, [online], available: http://www.designcouncil.org.uk/about-design/What-design-is-and-why-it-matters/ [Last accessed 08/03/2013].
HESKETT, J. (2002) Design: A Very Short Introduction, Oxford: Oxford University Press.
JONES, J. (1970) Design Methods: seeds of human futures, Chichester: John Wiley and Sons.
JONES, J. (1992) Design Methods, 2nd edn, New York: Wiley.
LAWSON, B. (2006) How designers think: the design process demystified, 4th edn, Oxford: Architectural Press.
PAPANEK, V. (1985) Design for the Real World: Human Ecology and Social Change, 2nd edn, Chicago: Academy Chicago Publishers.
PEIRCE, C. S. (1998) The essential Peirce, Vol. 2 (1893 – 1913), Bloomington: Indiana University Press.
SIMON, H. (1996) The sciences of the artificial, 3rd edn, Cambridge: The MIT Press.
VISSER, W. (2006) The cognitive artifacts of designing, Mahwah: Lawrence Erlbaum Associates.
Have you read Defining Design (Part I)?