A braze-on is the name for any number of parts of a bicycle

Bicycle

A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

 which have been permanently attached to the frame

Bicycle frame

A bicycle frame is the main component of a bicycle, on to which wheels and other components are fitted. The modern and most common frame design for an upright bicycle is based on the safety bicycle, and consists of two triangles, a main triangle and a paired rear triangle...

. The term "braze-on" comes from when these parts would have been brazed on to steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

 frame bicycles. Braze-ons continue to be so called even though they may be welded

Welding

Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

, glued

Adhesive

An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...

, rivet

Rivet

A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...

ed, or moulded into the frame material, depending on the material itself and the connection method used elsewhere on the frame.

Uses

Braze-ons include:

• Rack
  Luggage carrier
  A luggage carrier, also commonly called a rack, is a device attached to a bicycle to which cargo or panniers can be attached. This is popular with utility bicycles and touring bicycles....
  
   and fender mounts at the dropouts
  Dropout (bicycle part)
  A bicycle dropout is a type of fork end that allows the rear wheel to be removed without first derailing the chain. The more inclusive term fork end refers to a slot in a frame or fork where the axle of the wheel is attached....
  
  , seatstays, and fork
  Bicycle fork
  A bicycle fork is the portion of a bicycle that holds the front wheel and allows the rider to steer and balance the bicycle. A fork consists of two fork ends which hold the front wheel axle, two blades which join at a fork crown, and a steerer or steering tube to which the handlebars attach ...
  
   blades.
• Water bottle cage
  Bottle cage
  A bottle cage is device used to affix a water bottle to a bicycle. Composed of plastic, aluminum, stainless steel, titanium or carbon fiber, it is attached to the main frame of a bicycle, the handlebars, or behind the saddle...
  
   mounts.
• Cable
  Bowden cable
  A Bowden cable is a type of flexible cable used to transmit mechanical force or energy by the movement of an inner cable relative to a hollow outer cable housing...
  
   carriers, guides
  Cable guide
  A cable guide is a fitting or part of a bicycle frame which guides a piece of bare inner bowden cable around a corner. Most multi-speed bicycles have cable guides to get the derailleur cables past the bottom bracket....
  
  , and stops.
• Pump
  Bicycle pump
  A bicycle pump is a type of positive-displacement pump specifically designed for inflating bicycle tires. It has a connection or adapter for use with one or both of the two most common types of valves used on bicycles, Schrader or Presta...
  
   pegs.
• Shifter
  Shifter (bicycle part)
  A bicycle shifter or gear control or gear levers is a component used to control the gearing mechanisms and select the desired gear ratio. Typically, they operate either a derailleur mechanism or an internal hub gear mechanism. In either case, the control is operated by moving a cable that connects...
  
   bosses.
• Cantilever brake
  Bicycle brake systems
  A bicycle brake is used to slow down or stop a bicycle. There have been various types of brake used throughout history, and several are still in use today...
  
   bosses.
• Chain
  Bicycle chain
  A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are nickel-plated to prevent rust, or simply for aesthetics. Nickel also confers a measure of...
  
   hanger, inside the drive-side seatstay.
• Front derailleur hanger.
• Hub brake reaction arm mount. Called a Pacman braze-on if formed with a slot instead of a hole.