I believe that the all encompassing term "curry" was just an easy and simple way of naming Indian cuisine, on the part of the British. Another side of the imperialist ethic. Needless to say, it is an example of serious reductionism and oversimplification of matters that are complex and nuanced. What if you named the whole of Italian cuisine "marinara" ? ?
Or the whole of French cuisine, "bechamel" ? well, pretty dangerous, I should say!
I won't go into the political and historical motives or situations behind such things, deliberate or otherwise. I have read many versions of how the word came to be used. But I have formed my own theory.
Curry, for us, is any dish which is rather "saucy". By that I mean a dish which has a sauce, sometimes real watery, at other times, rather thick, and at some other times, in between. Curry is not a spice. Nor is curry and turmeric interchangeable. Turmeric is a spice. The curry powder that I see at the supermarket here, is really sambar powder. Sambar, being a South Indian vegetarian soup or stew, and sambar powder, being a mix of various spices, including turmeric, used in that dish. For cooking meat dishes, we have a completely different basic spice mix. As for fish. And the mix varies for different dishes in the same group, and it varies by region too, needless to say. So if you are using the so-called curry powder for everything, and then calling it Indian food, then, you are missing out on the real thing.
I imagine that "curry" for the British, came from "curry leaves", we call it kariveppila, (as opposed to "aryaveppila" which is the leaf of the neem tree). Kariveppila is a herb that we use profusely in our cooking. (By the way, it has medicinal properties like many other herbs. Kariveppila grows on "Kariveppu" , the tree. Back home, in Kerala, kariveppu grows lush and tall.. I still remember the trees in my grandmother's yard. I have been trying to grow one here, indoors. Still a baby -- looks like it will stay that way forever. forever young!
And what do we call "curry" ? In Malayalam, "koottaan" is the term we use. Literally it means 'to add". Something that we add and mix with rice. But then we also say "curry" when we mean a dish that is not dry, but has some sauce or gravy -- not pertaining to any spice, like I mentioned before.