Ontologies are not only crucial for extending the traditional web into the Semantic Web but also for developing intelligent applications, by converting the raw data into smart data, through semantic enrichment. However, crisp Ontologies are not able to represent fuzzy knowledge which is often encountered in real-world applications. Fuzzy Ontology introduces fuzzy logical rules in Ontology for representing imprecise domain concepts such as darkness, hotness, thickness, creamy etc. in a machine-readable and interoperable format. The performance of fuzzy Ontology decreases with the increase of fuzziness in the domain knowledge. Type-2 fuzzy Ontologies (T2FO) were introduced to represent the domain knowledge where the concepts are either extremely vague or their vagueness increases gradually. The type-2 fuzzy Ontology domain is continuously expanding and there is a need to provide a comprehensive review incorporating the literature of T2FO development approaches, its applications in different domains, reasoners developed for inferencing on type-2 fuzzy Ontology, and evaluation approaches. To perform a comprehensive survey about the T2FO, we used Google Scholar as the main literature research tool to review papers published between 1998 to 2018. We then summarized the published approaches by comparing their features proposed for T2FO development, reasoning or inference, and evaluation approaches. This paper also identifies the domains wherein the past T2FO has been used to develop real-world applications. We conclude this paper by summarizing the previous work, and by identifying the research gaps for investigators.

中文翻译：

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2 类模糊本体的综合回顾

本体不仅对于将传统 Web 扩展到语义 Web 至关重要，而且对于开发智能应用程序（通过语义丰富将原始数据转换为智能数据）也至关重要。然而，清晰的本体无法表示现实世界应用中经常遇到的模糊知识。Fuzzy Ontology 在 Ontology 中引入了模糊逻辑规则，用于以机器可读和可互操作的格式表示不精确的领域概念，例如黑暗、热度、厚度、奶油等。模糊本体的性能随着领域知识模糊度的增加而降低。引入了类型 2 模糊本体 (T2FO) 来表示概念非常模糊或模糊度逐渐增加的领域知识。2 类模糊本体领域正在不断扩展，需要提供全面的综述，包括 T2FO 开发方法的文献、其在不同领域的应用、为推理 2 类模糊本体而开发的推理器以及评估方法。为了对 T2FO 进行全面调查，我们使用 Google Scholar 作为主要文献研究工具来回顾 1998 年至 2018 年间发表的论文。然后我们通过比较它们为 T2FO 开发、推理或推理提出的特征以及评估方法来总结已发表的方法. 本文还确定了过去使用 T2FO 开发实际应用程序的领域。我们通过总结以前的工作并确定研究人员的研究差距来结束本文。