I don't think I would call 肥肉 "fat", at least not in isolation. "Fatty meat" is probably the best translation, though it still feels imperfect.
The only circumstance I can think of where 肥肉 might be called just "fat" would be in direct contrast to "lean", and even then the context would have to be quite clear. One example is in the nursery rhyme Jack Sprat ("Jack Sprat could eat no fat, his wife could eat no lean. And so between the two of them, they licked the platter clean.") but I think that usage is a bit old-fashioned now — I'd be surprised to see it in anything modern. (I wouldn't say just "lean", either, I'd always say "lean meat".)
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The only circumstance I can think of where 肥肉 might be called just "fat" would be in direct contrast to "lean", and even then the context would have to be quite clear. One example is in the nursery rhyme Jack Sprat ("Jack Sprat could eat no fat, his wife could eat no lean. And so between the two of them, they licked the platter clean.") but I think that usage is a bit old-fashioned now — I'd be surprised to see it in anything modern. (I wouldn't say just "lean", either, I'd always say "lean meat".)
How about "rendered beef fat" for 牛油?