The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The chainrings and pedals attach to the cranks. Bottom bracket bearings fit inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.

The term "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames which also form the shell that contains the spindle and bearings; the term is now used for all frames, bracketed or not.

There is some disagreement as to whether the word axle or spindle should be used in particular contexts. The distinction is based on whether the unit is stationary, as in a hub, or rotates, as in a bottom bracket. American bicycle mechanic and author Sheldon Brown uses axle once and spindle four times in his bottom bracket glossary entry. This article uses spindle throughout for consistency.

Bottom bracket assemblies are available in several types, and can be split into whether they are assembled and disassembled with screw threads or whether they are pressed into the bottom bracket. Since the 2000s and especially the 2010s, a lack of standardization, or rather the constant introduction of new standards that disappear after relatively short periods, has been described as a complex topic to deal with for those who want to buy bicycle components or maintain bicycles. Many bicycle brands have introduced their own dimensions for bottom bracket bearings, and the different use of terminology by the various manufacturers has been described as confusing.

An old American term for the bottom bracket is hanger. This is usually used in connection with Ashtabula cranks, alternatively termed one-piece cranks.

In typical modern utility bikes, the spindle is separate from the cranks. This is known as a three-piece crankset (spindle, left crank-arm and right crank-arm). The cranks attach to the spindle via a common square taper, cotter, or splined interface.

Earlier three-piece cranks consist of a spindle incorporating bearing cones (facing out), a fixed cup on the drive side (with a cone), an adjustable cup on the non-drive side (also with a cone), and loose bearing balls (or held by a cage). Overhauling requires removing at least one cup, cleaning the cups, cleaning or (more usually) replacing the bearing balls, reinstalling the spindle, and adjusting the cups. The design is nowadays mostly found on affordable bikes due to its simple and affordable design, and is also an easily serviceable design.

The Bayliss Wiley unit bottom bracket is a self-contained unit that fits into a plain, slightly larger-than-usual bottom bracket in a bicycle frame. It comprises a standard spindle and bearings in a steel cylinder with a slightly-modified bearing cup at each end. The cylinder, bearing, and spindle are placed in the shell and held in place by the bearing cups, each of which has a narrow flange that bears against the edge of the shell.