Widget: Widgets are the individual components for creating user interfaces in FreeCAD. Widgets can display data and status information, receive user input, and provide a container for other widgets that should be grouped together. FreeCAD uses the QtWidget toolkit.

Element: A primary component of the FreeCAD User Interface consisting of a combination of widgets to support navigation and feature execution providing a means for user interaction.

Panel: A window element which can be docked to the edge of the screen in a static position.

Main View / 3D View: This is the main view area of the users screen where work is displayed and these two terms are largely interchangeable within FreeCAD.

Contrast: Difference between colors. For accessibility, contrast refers strictly to the difference in tone. A difference of 40 in tone guarantees a WCAG contrast ratio ≥ 3.0; a difference of 50 in tone guarantees a contrast ratio ≥ 4.5. FreeCAD makes extensive use of colors and high contrast ratios should be a developers goal.

Design guidelines: Guidelines are descriptive written and illustrated docs that demonstrate usage and behavior mainly through examples. They’re the long-form discussion of specs that help designers and developers with problem-solving and decision-making.