Knowledge Organization Systems (KOS), such as term lists, thesauri, taxonomies, and ontologies, play a fundamental role in categorising, managing, and retrieving information. In the academic domain, KOS are often adopted for representing research areas and their relationships, with the primary aim of classifying research articles, academic courses, patents, books, scientific venues, domain experts, grants, software, experiment materials, and several other relevant items and agents. These structured representations of research areas have been adopted by most academic fields and proved very effective in i) improving the retrievability of relevant documents, ii) enabling advanced analytic solutions to quantify the impact of academic research, and iii) understanding and forecasting research dynamics. The objective of this paper is to present a comprehensive survey of the current KOS for academic disciplines. We defined formal inclusion and exclusion criteria that led to the identification of 44 KOS. We analysed and compared them according to five main dimensions: scope, structure, curation, usage, and links to other KOS. Our results reveal a very heterogeneous scenario in terms of scope, scale, quality, and usage, highlighting the need for more integrated solutions for representing research knowledge across academic fields. We conclude by discussing the main challenges and the most promising future directions.