From the EU's COST website:
"A COST Action is an interdisciplinary research network that brings researchers and innovators together to investigate a topic of their choice for 4 years. COST Actions are typically made up of researchers from academia, SMEs, public institutions and other relevant organisations or interested parties.
Open to all science and technology fields, including new and emerging fields; COST Actions offer an inclusive, pan-European environment for individuals of all levels of seniority to grow their professional research networks and boost their careers.
In today’s world it is essential for research to be interconnected, interdisciplinary, collaborative and data-intensive. By focusing on networking activities, COST Actions help to advance knowledge and strengthen the research and development sector, by creating networking opportunities for researchers to meet and discuss ideas, complex problems can be addressed in a targeted way, across a large geographical area."
For more information: What are COST Actions?