A design is a plan or specification for the construction of an object or system or for the implementation of an activity or process, or the result of that plan or specification in the form of a prototype, product or process. The verb to design expresses the process of developing a design. In some cases, the direct construction of an object without an explicit prior plan (such as in craftwork, some engineering, coding, and graphic design) may also be considered to be a design activity. The design usually has to satisfy certain goals and constraints, may take into account aesthetic, functional, economic, or socio-political considerations, and is expected to interact with a certain environment. Major examples of designs include architectural blueprints, engineering drawings, business processes, circuit diagrams, and sewing patterns.^[1]

The person who produces a design is called a designer, which is a term generally used for people who work professionally in one of the various design areas—usually specifying which area is being dealt with (such as a fashion designer, product designer, web designer or interior designer), but also others such as architects and engineers. A designer’s sequence of activities is called a design process, possibly using design methods. The process of creating a design can be brief (a quick sketch) or lengthy and complicated, involving considerable research, negotiation, reflection, modeling, interactive adjustment and re-design.

The rational model was independently developed by Herbert A. Simon,^[8][9] an American scientist, and two German engineering design theorists, Gerhard Pahl and Wolfgang Beitz.^[10] It posits that:

1. Designers attempt to optimize a design candidate for known constraints and objectives.
2. The design process is plan-driven.
3. The design process is understood in terms of a discrete sequence of stages.

The rational model is based on a rationalist philosophy^[4] and underlies the waterfall model,^[11] systems development life cycle,^[12] and much of the engineering design literature.^[13] According to the rationalist philosophy, design is informed by research and knowledge in a predictable and controlled manner.

At least two views of design activity are consistent with the action-centric perspective. Both involve three basic activities.

In the reflection-in-action paradigm, designers alternate between “framing“, “making moves”, and “evaluating moves”. “Framing” refers to conceptualizing the problem, i.e., defining goals and objectives. A “move” is a tentative design decision. The evaluation process may lead to further moves in the design.^[5]

In the sensemaking–coevolution–implementation framework, designers alternate between its three titular activities. Sensemaking includes both framing and evaluating moves. Implementation is the process of constructing the design object. Coevolution is “the process where the design agent simultaneously refines its mental picture of the design object based on its mental picture of the context, and vice versa”.^[6]

The concept of the design cycle is understood as a circular time structure,^[20] which may start with the thinking of an idea, then expressing it by the use of visual or verbal means of communication (design tools), the sharing and perceiving of the expressed idea, and finally starting a new cycle with the critical rethinking of the perceived idea. Anderson points out that this concept emphasizes the importance of the means of expression, which at the same time are means of perception of any design ideas.^[