The term "bottom bracket" refers to a part of a bicycle that holds the crankset and allows it to rotate smoothly. It is located at the bottom of the bike frame, between the two pedals. Full definition
We used two cranks and two bottom brackets in the front to gear up the ratio. (treehugger.com)
It also offers a broad range of complementary stainless components, including the integrated cups and BB30 bottom bracket shell found on Cinelli's XCr. (treehugger.com)