Is it the flow diagram, User Interface or what?
From wikipedia:
In systems, design functions and operations are described in detail, including screen layouts, business rules, process diagrams and other documentation. The output of this stage will describe the new system as a collection of modules or subsystems. The design stage takes as its initial input the requirements identified in the approved requirements document. For each requirement, a set of one or more design elements will be produced as a result of interviews, workshops, and/or prototype efforts. Design elements describe the desired software features in detail, and generally include functional hierarchy diagrams, screen layout diagrams, tables of business rules, business process diagrams, pseudocode, and a complete entity-relationship diagram with a full data dictionary. These design elements are intended to describe the software in sufficient detail that skilled programmers may develop the software with minimal additional input.
http://en.wikipedia.org/wiki/Systems_Development_Life_Cycle#Design
In the design phase of the SDLC, making solution is focused via system modeling. Uml modeling is performed which is called the Unified modeling language. UML modeling has the following steps
- Class diagram
- Use case diagram
- Collaboration diagram
- Sequence diagram
The UML modeling is basically for defining the problem or we can say it is a tool to solve the problem.
More : http://websolace.net/web-design-solutions/sdlc-maintains-the-quality/