Complete data representing the relevant nuclear physics are required for the simulation of nuclear systems. Collectively, the discipline is known as nuclear data. These data must be stored in a standardised format and, since the release of the American ENDF/BVI library and its associated ENDF-6 format in 1990, the ENDF-6 format has become a de facto worldwide standard. This format is based on legacy technology from an era when computers utilised punch-card readers and as a result the formats introduce artificial limitations, require legacy programming techniques to interface with the data and force new scientists and engineers to learn numerous Byzantine rules. The NEA Working Party on International Nuclear Data Evaluation Co-operation (WPEC) was created in 1989, under the aegis of the Nuclear Science Committee (NSC), to improve the quality and completeness of nuclear data by bringing together representatives of the major nuclear data evaluation projects of NEA member countries and of selected invitees. The WPEC community recognised the need for a new format that embraced modern computer programming paradigms, could address more sophisticated user requirements and was maintained by the international community. Following the success of the WPEC subgroup 38 “Beyond the ENDF format: A modern nuclear database structure,” which prepared the requirements for a replacement of the ENDF-6 format, WPEC established the Expert Group on the Recommended Definition of a General Nuclear Database Structure (GNDS). This Expert Group is tasked with defining the specifications of the GNDS, formalising these specifications and publishing them through the NEA. Multiple versions of the GNDS specifications have been developed in parallel to the development of the requirements in an iterative process. The first public Version 1.9, which served effectively as a one-to-one translation of the ENDF-6 format as defined at the release of the 2018 ENDF/B-VIII.0 library, was published by the NEA in 2020. GNDS Version 2.0, published in 2023, was a major update with 149 formally approved change requests made by the NEA Expert Group. This document contains the specifications for Version 2.1, which serves as a modest update with several formally approved change requests compared to version 2.0. Changes in the latest version focus on improving handling for thermal neutron scattering law data, along with many fixes for inconsistencies and errors in the previous specification document.