Terminology index - a list of bike part names and cycling concepts This question and its answers list the names of bike parts and cycling concepts.
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Make sure you only put one term per answer!
Try to include an image if applicable
Include sources that contain detailed information
Add a link to the index in this question using edit.

Also, I made this a community wiki, so that anyone will be able to edit it, and to stop rep-hoarding

There's a handy reference at the Park Tool Co. website, a bike repair map; it's a diagram of a bike with all the parts labeled, and is very handy! At the moment, the diagram is up at parktool.com/blog/repair-help. (They've changed the URL in the past, so this link may break.)
A road bike has the following parts (source):

A mountain bike has the following parts (source):


Edit: This page is meant to identify what things or concepts are (as per this thread in meta). If you want to recommend an accessory or a specific product you've found handy, please use the accessories page.





Contents





Axle
Axle Nuts


BCD (Bolt Circle Diameter)BearingBonk/BonkingBottle Cage / Bottle HolderBottom Bracket
Boom/Boom TubeBrazed FrameBrifterBSD (Bead Seat Diameter)BSO/Bike-Shaped-Object


Cable PullCable StretcherCadenceChain GaugeChain Guard/CoverChain ToolChain Tug/Chain Tensioner
Chainstay LengthChainsuckChamoisClipless PedalsCoaster Brake (foot brake / pedal brake)Crank


DerailleurDerailer Hanger/Derailleur RangerDisc HubDoor Zone
DropoutDropper PostDunlop Valve


e



Fender/Mudguard/MudflapsFixed-GearFlip-Flop Hub
Folding BikeFrame


Gear Inches
Groupset


HandlebarsHeadsetHose Clamp aka Jubilee Clip
HubHub Skewer


Internally-Geared Hub



j



Keel Tube



Lawyer lips/lawyer tabsLateral TubeLBS/Local Bike Shop
Luggage Carrier/RackLugged Frame


Master LinkMixte
Mountain Bike


n



o



PannierPlayPower Meter
Presta Valve/Presta TubePump Peg


Q-Factor
Quick-Release


REI (Recreational Equipment Inc)
RimRim Tape


SaddleSaddlebagSchrader Valve/ Schrader TubeSingle-speedSkewer
SpiderSpokeStemSuspension Fork/Rear Shock


Through/Thru AxleTire ClearanceTire Lever/Tire Iron
Tire SaverTrack Pump/Floor PumpTriathalon Bars/Triathlon Bars


u



v



Welded Frame



x



y



z
Bicycle-Shaped Object (BSO)
A derogatory term for a very cheaply produced bike with very low quality components. The components can be hard to maintain due to poor tolerances. BSOs are often sold at non-specialty retail stores.
The etymology of the term is uncertain. It appears in use in some parts of the English-speaking world. It may originate in the UK, as discussed at the link. Other languages may use terms equivalent to supermarket bike or department store bike. These terms are likely to be understood in English as well.

For instance the BSO pictured is being sold in the UK by ASDA (owned by Walmart) for £75. These bikes tend to be mass produced and sold in flat pack boxes for self-assembly.
Purchasers of lower-end bicycles tend to be less experienced cyclists who focus on price. Many BSOs carry features which are included for marketing purposes but are unnecessary for the typical end-user. Such features may include front and rear suspension, wide off-road style tyres and an excessive number of gear ratios. For cyclists who are riding on city streets or smooth trails, these features are unnecessary. Including these features reduces the budget available for better components elsewhere.
It is more advisable to search for a cheap second-hand bike in a similar price range from a more experienced cyclist or on eBay than to go for one of these.