Final answer:
In the sentence provided, 'converse' is used as an adjective meaning the opposite or reverse of the initial statement, indicating that not all space travelers are trained astronauts.
Step-by-step explanation:
In the sentence 'All trained astronauts are space travelers, but the converse, that all space travelers are trained astronauts, is not true.', the word converse refers to the opposite or reverse of something previously mentioned. In this context, it means that while the first statement asserts that every astronaut is a space traveler, the reverse (converse) statement implying that every space traveler must be a trained astronaut is not accurate. This use of converse falls under its definition as an adjective.