The phrase "decisive role" means that something or someone has a very important and influential impact on the outcome of a situation or decision. Full definition
A particularly decisive role in this process is the interactive communication with surrounding tissue cells in the bone marrow. (sciencedaily.com)
We often take a prominent and decisive role in negotiations to effect a swift and effective conclusion to fundraising transactions. (clarkslegal.com)
The concept of the trifecta is important in state lawmaking because in many states, the governor, senate majority leader, and house majority leader play decisive roles in the legislative process. (ballotpedia.org)