The technical debt metaphor is used to describe the long-term consequences of engineering decisions made to achieve a short-term benefit. The metaphor originated in the field of software engineering and has begun to migrate to other fields, including systems engineering. The usage of the metaphor, its associated terminology, and basic definitions vary both within the software field and within the greater engineering community. The lack of consistent definitions inhibits the ability of system developers to understand and control technical debt within their system developments. This paper presents an ontology for technical debt, focusing on the field of systems engineering. By providing a set of concise and consolidated definitions, this ontology enables precise discussion of technical debt and associated techniques for mitigating its impact within systems engineering.