I've gathered that hundreds of years ago, the concept of "science" was known as "philosophy" - for instance, what we would call biology was "natural philosophy" and so forth. This seems to have persisted well into the 19th century. When and why did the terminology finally change?
and how did the terms end up practically opposed, with philosophy becoming a purely humanities discipline that has sort of a reputation as being "irrelevant" (quotes because I personally disagree) to scientific disciplines and human progress?
The two terms coexisted for millennia. The early Greek natural philosophers were trying to explain why the world is the way it is, and how everything within it functions. It started with identifying the four elements (earth, air, fire, and water) and the outcomes of blending those four elements.
Then some natural philosophers started analyzing discrete bodies and how they function. This is around the time you get Hippocrates and the earliest physicians. But even at this time, they're still interested in a holistic view of bodies in their natural world. They're still influenced by philosophical analysis.
Physicians become gradually more and more sophisticated, and in the 3rd century BC in Alexandria, Herophilus and Erasistratus turn that corner. They strictly pursue analysis via scientific method. They're employing experiments and analysis that can be replicated. Their work has nothing to do with man in his environment. It's the birth of science, really.
From there, science involved technicians, whereas a more anthropological analysis of human interaction stayed in philosophy.