These are common terms amongst the MTB'ers, and some are the same for BMX. Many BMX'ers ride MTB too.
Bail: Leaping off the bike to avoid a more serious crash. Best done into a soft pile of leaves at the side of a trail.
Berm: A banked corner that can be ridden faster than a flat corner. A very common trail feature.
Booter: A large jump that requires a lot of commitment.
BSO: Bike Shaped Object. A cheap bike designed to look like a mountain bike that would have no real off-road use. Usually dept store bikes.
Clean: To complete a section of trail without crashing, stopping or taking your feet off the pedals.
Dab: Quickly taking a foot off your pedal to stop yourself from crashing.
Dialled: When your set up is perfect allowing you to ride to the top of your ability.
Dope: Good.
Downside: A downwards-facing slope that allows you to gain extra speed, it will normally be after a jump.
Edit: A short film showcasing the talents of a rider or riders
Flow: The trail nirvana. A feeling all mountain bikers seek where one obstacle melds into another just perfectly. You know it when you’ve found it.
Gap: A jump with a hole in the middle, this increases the risk for the rider.
Gnarly: An especially difficult feature.
Huck: Performing a large jump without any real thought for the consequences.
Kicker: A steep jump that gives you a lot of airtime.
LBS: Local Bike Shop. Your go-to place for any repairs, upgrades or just a nice chat about bikes.
Loam: A specific type of loose, dry dirt. Desirable for its grippy characteristics and the ability to create roost.
Loose: To ride on the edge of control.
Northshore: Raised wooden board walks, named after the North Shore area of Vancouver that popularized this style of riding.
Pimp: Extremely nice bikes or components.
Pinned: To ride fast
Pump: A technique that allows you to gain speed without pedaling.
Rad: Good.
Rail: To ride a corner so well it is as if you are on rails.
Roost: Dirt that is kicked up behind a rider as they ride sideways into a corner.
Scrub: A motocross technique used to keep low and fast over a jump.
Session: To repeatedly ride a section until you have perfected it.
Shralp: To ride in an aggressive manner.
Shred: To ride in an aggressive manner.
Sick: Good
Snake Bite: A puncture that leaves two parallel holes like a snake’s fangs
Step-down: A jump where the landing is lower than the take-off.
Step-up: A jump where the landing is higher than the take-off.
Stoked: Excited.
Stoppie: A nose wheelie.
Tabletop: A jump with a flat layer of dirt across the top, this is thought to be safer than a double or gap jump.
Also, a trick where the bike is laid flat underneath the rider in the air.
Taco: When a wheel has been bent by an impact to the extent that it looks like a taco.
Whip: When the bike is pushed sideways in the air.
Yew: A general expression of excitement. Sam Pilgrim’s favorite riding word… yyyeeeewwwww!