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A ring shaped like a D that's stuck to a collar or belt so you can attach stuff to it, like a chain or a leash or something that smells like old pizza.
My dog's collar has a d-ring so I can attach his leash and his chew toy at the same time.
My mom used a d-ring to attach a bell to my brother's pants so he'd ring every time he walked.
The guy at the gym had a d-ring on his belt so he could attach his dumbbells and his snack.