Adjustable Mountain Bike Seat Post

mountain bike
a bicycle with a sturdy frame and fat tires; originally designed for riding in mountainous country
(Mountain biking) Mountain biking is a sport which consists of riding bicycles off-road, often over rough terrain, using specially adapted mountain bikes. Mountain bikes share similarities with other bikes, but incorporate features designed to enhance durability and performance in rough terrain.
(Mountain Biking) A designated, rugged, natural surfaced, single track trail that offers a range of riding opportunities.
A bicycle with a light sturdy frame, broad deep-treaded tires, and multiple gears, originally designed for riding on mountainous terrain

adjustable
(Adjustables) Fixtures that can be adjusted or aimed for accent and directional lighting requirements.
Able to be adjusted
capable of being changed so as to match or fit; "adjustable seat belts"
(Adjustability) how well you can adjust the nappy or wrap to fit your baby. A good fit is vital to achieve maximum containment, especially in a newborn, breastfed baby.

seat post
A bicycle seatpost, seatpin, or saddlepole is a tube that extends upwards from the bicycle frame to the saddle. The amount that it extends out of the frame can usually be adjusted, and there is usually a mark that indicates the minimum insertion (or maximum extension).
Frame component onto which the saddle is attached. The seatpost is inserted into the seat tube.
The removable and adjustable (sometimes chrome) tube that the saddle mounts onto. See photo below.