1. A border or boundary between territories, regions, or concepts that has not been clearly established, demarcated, or determined
The undefined frontier between the two countries led to frequent disputes.
A fronteira indefinida entre os dois países levou a frequentes disputas.
2. An area of uncertainty or unclear limits in knowledge, exploration, or jurisdiction
Scientists face an undefined frontier in quantum physics research.
Os cientistas enfrentam uma fronteira indefinida na pesquisa de física quântica.