Understanding Non-Functional Requirements (NFR) and their solutions is a key architecture task, and reusing past solutions is an effective and efficient way to satisfy NFRs. NFR catalogues have been proposed to store and reuse NFR knowledge descriptions, yet identification of reusable knowledge inside catalogues remains quite hard and tedious. This article introduces the NDR Ontology to describe NFR and Design Rationale knowledge, thus making it reusable and shareable via human and machine-readability The ontology is the basis for several semantics-based applications currently under development, including view-based NFR/rationale exploration, facet-based search, and comparison of alternative solutions. Also, some approach extensions are explored, including model merging and high-end visualization of NFR knowledge.