A Johnson bar is a hand lever with several distinct positions and a positive latch to hold the lever in the selected position. The latch is typically activated with a spring-loaded squeeze handle on the lever so that only one hand is needed to release the latch, move the lever, then re-engage the latch in a different position. This is an American English term, in British English the lever is named for its function.
Many steam locomotives have valve gear controls which are set using a Johnson bar as referenced in Fred Eaglesmith's Back There: Hey Porter, tell that engineer, tell him this train's too slow. Tell him to let go that Johnson bar. I got places I got to go.
Many trucks and buses use a hand brake which is controlled with a Johnson bar. These are sometimes called "Johnson bar brakes".
Truck drivers used to call lever controls on air-operated trailer brakes "Johnson bars".
On Caterpillar tractors the forward/reverse lever is also called a Johnson bar.
Some light general aviation aircraft (including Piper Cherokees, Beech Musketeers, and some early model Cessnas) use Johnson bars to actuate flaps and wheel brakes; a small number of older aircraft (including the Mooney M-18 and some older M20s) also have landing gear actuated by Johnson bars.
On the Boeing 707/720 aircraft the Johnson bar was used to manually extend the nose landing gear. This was only used if the normal gear extension failed.
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