In Japanese, usamimi うさみみ means "bunny ears" or "rabbit ears." It's a portmanteau combining the words usagi 兎, "bunny," "rabbit," and mimi 耳, "ears." In other words: it's an abbreviation of usagi-mimi 兎耳.
Like "cat ears," nekomimi 猫耳, usamimi is a specific type of kemomimi ケモミミ, and normally refers to headbands featuring bunny ears, or characters featuring bunny ears, rather than the actual ears of a bunny.
Anime: K-On!, Keion! けいおん！ (Episode 9)