A new user requested help with acronyms often used in mountainbiking. This is by no means a complete listing, and can be added to by other users here.
MTB:
Mountainbike
XC:
Cross-Country- May refer to a mountainbike or race. XC bikes tend to have less than 4" of suspension travel and may be hardtail or full-suspension.
DH:
Downhill
AM:
All-Mountain
FR:
Freeride
FS:
Full-Suspension; also may be referred to as "Full-Susser", which is often used by UK riders. "Squishies", "Dual-Bounce", & "Double-Squish" may also be used in referrence to a full-suspension mountainbike.
Hardtail:
Mountainbike with no rear suspension, but may have a suspension fork.
Softtail:
Mountainbike with a flexible rear triangle, most often offering less than 2" of travel.
Rigid:
A mountainbike with no suspension.
SS:
Single-speed; a mountainbike with only one available gear.
29'er:
A mountainbike with 29" wheels. Frequently found as single-speed and rigid variations.
69'er:
A mountainbike with one 26" rear wheel, and one 29" front wheel. Also a name for a mountainbike with these features by Trek.
650B:
A new mountainbike wheel that is 27.5". More often found on touring or city bicycles in Europe.
BB:
Bottom-Bracket; or the cups, bearings, & axle/spindle that the cranks attach to and rotate on.
Front Triangle:
The shape of a bicycle frame section described by the junctions of the head-tube, down-tube, seat-tube, and top-tube.
Rear Triangle:
The shape of a bicycle frame section described by the junctions of the seat-tube, chain-stays, drop-outs, and seat-stays.
Seat-Stay:
The section of frame that runs from the seat-tube down to the rear dropouts.
Chain-Stay:
The section of frame that runs from the Bottom-Bracket to the rear dropouts.
Dropouts:
The section of frame into which the wheel axles slide into. Bicycle forks also have dropouts, into which the front-wheel axle is fitted. May be horizontal or verticle, depending on the type of bicycle. Horizontal dropouts are most commonly found on SS mountainbikes, BMX bikes, and city/touring bicycles. Verticle dropouts are found most commonly on any bicycle with gears.
Hanger; Derailleur Hanger:
A small part that is threaded to accept a rear derailleur, and is fitted to the rear triangle.
Head-Tube:
A short section of tubing through which the fork steer-tube runs.
Head-Tube Angle:
The angle at which the headtube is situated. This angle is also described as part of a bicycle's "Geometry".
Seat-Tube Angle:
The angle at which the seat-tube is situated. May be anywhere from 58 degrees to 74 degrees. This angle is also described as part of a bicycle's "Geometry".
Geometry:
The angles at which a bicycle's frame tubes are set. These angles affect the handling of the bicycle. Smaller angles are described as being "slack", and are often used on DH or FR mountainbikes. Larger angles are described as being "steep", and are often used on XC mountainbikes.


