Form, fit and function

Form, Fit, and Function (F3 or FFF) is a concept used in various industries, including manufacturing, engineering, and architecture, to describe aspects of a product's design, performance, and compliance to a specification. F3 refers to a set of characteristics or requirements that are essential for the design and compatibility of products, components, or systems, and can have legal considerations in regulated industries like aviation and defense (e.g., for technical data rights and configuration management).

The concept originates in the 1960s, and in some cases as form-fit-function. The United States (US) Government formally recognized it in the legal incorporation of Public Law 98-525 regarding technical data and design changes. F3 can also refer to the ability of a replacement unit or technology upgrade to be compatible with existing systems, or be compatible with change control procedures (e.g., NASA's use in reliability via military standards).

Alternate Uses
Some organizations have supplemental considerations for F3. The United States Navy has been using Form, Fit, Function, and Interface (F3I) since the 1970s, and NASA has published references to Form, Fit, Function, and Reliability to facilitate reliable designs.

In the Code of Federal Regulations - US Government
Source:

Form The form of a commodity is defined by its configuration (including the geometrically measured configuration), material, and material properties that uniquely characterize it. For software, the form means the design, logic flow, and algorithms.

Fit The fit of a commodity is defined by its ability to physically interface or connect with or become an integral part of another commodity. For software, the fit is defined by its ability to interface or connect with a defense article.

Function The function of a commodity is the action or actions it is designed to perform. For software, the function means the action or actions the software performs directly related to a defense article or as a standalone application.

By Clickfold Plastics, Inc.
Fit refers to the ability of the part or feature to connect to, mate with, or join to another feature or part within an assembly. The “fit” allows the part to meet the required assembly tolerances to be useful.

Form refers to such characteristics as external dimensions, weight, size, and visual appearance of a part or assembly. This is the element of F3 that is most affected by an engineer's aesthetic choices, including enclosure, chassis, and control panel, that become the outward "face" of the product.

Function is a criterion that is met when the part performs its stated purpose effectively and reliably. In an electronics product, for example, function can depend on the solid-state components used, the software or firmware, and quite often on the features of the electronics enclosure selected. Poorly placed or sized ports and misleading or missing labeling are two of the most common ways in which an enclosure can fail the function criterion.

Note on Source
The above definitions were modified from the original source, which is no longer available from the original webpage. Also, the company's new website no longer hosts their own F3 definitions.