A Functional Architecture is an architectural model that identifies system function and their interactions. It defines how the functions will operate together to perform the system mission(s).
Definition: A functional architecture is a set of functions and their sub-functions that defines the transformations of input flows into output flows performed by the system to achieve its mission. (SEBoK)
Types of Architectures
There is normally more than one architecture that can satisfy the requirements. Usually, each architecture and its set of associated allocated requirements have different cost, schedule, performance, and risk implications. The two most common architectures a system will have are:
- Functional Architecture
- Physical Architecture.
Purpose of Functional Architecture
The purpose of the functional architecture is to support functional and performance test development. It also supports system development, along with the physical architecture, of verification tasks that are defined to verify the functional, performance, and constraint requirements.
During the Functional Analysis and Allocation step, the functional requirements identified in the Requirements Analysis step are decomposed and their associated performance requirements into sub-functions to the point that they can be unambiguously related to the system elements or products that make up the design that flows out of a later step. The result is often called functional architecture. 
SMC Systems Engineering Handbook, Figure 15
In the Design Loop, synthesized designs are compared with the originating architectures and allocated requirements to assure compliance or to initiate re-evaluation. Sometimes it is necessary to drive toward optimal solutions by presenting various functional views including those that depict functional relationships with existing assets to enable more thorough assessments of plausible solutions. 
- The DoDAF descriptions in this website are very generic and are mostly taken from the DoDAF Architecture Framework website. Make sure you visit the actual website for the most update information and a more thorough explanation of each viewpoint.
- DoDAF Version 1.0, although outdated, has some good examples on how to construct AV’s, OV’s, and SV’s.
AcqLinks and References:
-  DoDAF Architecture Framework Version 2.02
- DoD Architecture Framework Working Group Version 1.0, Volume 1: Definition and Guideline, 9 Feb 04 (Old Version)
- DoD Architecture Framework Version 1.0, Volume 2: Product Description, 9 Feb 04 (Old Version)
- Website: DoDAF Architecture Framework – DoD Deputy Chief Information Officer
- Website: DoDAF Version 2.02 Journal
- Website: DoDAF Meta Model (DM2)
- Website: DoD Information Enterprise Architecture
- Website: OMB Enterprise Architecture Assessment Framework (EAAF)