Modular Open Systems Approach: an integrated business and technical strategy that: 

(A) Employs a modular design that uses major system interfaces between a major system platform and a major system component, between major system components, or between major system platforms; 

(B) Is subjected to verification to ensure major system interfaces comply with, if available and suitable, widely supported and consensus-based standards; 

(C) Uses a system architecture that allows severable major system components at the appropriate level to be incrementally added, removed, or replaced throughout the life cycle of a major system platform to afford opportunities for enhanced competition and innovation while yielding-  

(i) Significant cost savings or avoidance;  

(ii) schedule reduction;  

(iii) opportunities for technical upgrades;  

(iv) increased interoperability, including system of systems interoperability and mission integration; or  

(v) other benefits during the sustainment phase of a major system; and 

(D) Complies with the technical data rights set forth in Sec 2320, title 10.  [10 U.S.C. 2446a. (b) Sec. 805 (FY 2017 National Defense Authorization Act Modular Open Systems Approach (MOSA) definition for DoD Acquisition, Dec 2016)] 

 Open Systems Approach:  

(A) Employs a modular design and uses widely supported and consensus-based standards for key interfaces; 

(B) Is subjected to successful validation and verification tests to ensure key interfaces comply with widely-supported and consensus-based standards; and 

(C) Uses a system architecture that allows components to be added, modified, replaced, removed, or supported by different vendors throughout the life cycle of the system platform to afford opportunities for enhances competition and innovation, while yielding-   

     (i) Significant cost savings or avoidance; and (ii) increased interoperability. 

 [P.L. 113-291 Sec. 801 (FY15 NDAA Open Systems Approach (OSA) definition, Dec. 2014)] (DAU and Tri-Service Tiger Team) 

Modular Design: a design (organization) where functionality is partitioned into discrete, cohesive, and self-contained units with well-defined, open and published interfaces that permit substitution of such units with similar components or products from alternate sources with minimum impact on existing units. (A Modular Open Systems Approach (MOSA) to Acquisition, Open Systems Joint Task Force) 

Open Architecture: a technical architecture that adopts open standards supporting a modules, loosely coupled and highly cohesive system structure that includes publishing of key interfaces within the system and full design disclosure. (DAU Glossary) 

Open Standards: widely accepted and supports standards set by recognized standards organizations or the marketplace.  These standards support interoperability, portability and scalability and are equally available to the public under fair and reasonable license terms. (DAU Glossary) 

Open System: a system whose technical architecture adopts open standards and supports a modular, loosely coupled, and highly cohesive system structure.  This modular open architecture includes publishing of key interfaces within the system and relevant design disclosure. (DAU Glossary) 

Open System Architecture: a characteristic of a system which uses a technical architecture that adopts consensus based standards supporting a modular, loosely coupled, and highly cohesive system structure that includes the publishing of key interfaces within the system and relevant design disclosure. (DAU Glossary) 

Open System Acquisition of Weapons Systems: an integrated and business strategy that defines key interfaces for a system (or a piece of equipment under development) in accordance with those adopted by formal consensus bodies (recognized industry standards' bodies) as specifications and standards, or commonly accepted (de facto) standards (both company proprietary and non-proprietary) if they facilitate utilization of multiple suppliers. (DAU Glossary) 

Open Interface: a public standard for connecting hardware to hardware and software to software. With regard to hardware, it implies that there is more than one brand of product that can be hooked up to the device with the open interface. In the case of software, it implies that more than one program exists to interface with the application that has the open interface or that a program can be readily written to communicate with it. (OSA Contract Guidebook for Program Managers, v1.1, June 2013) 

Closed Interface: a privately controlled system/subsystem boundary descriptions that are not disclosed to the public or are unique to a single supplier. (DAU Glossary)