I am trying to build a vehicle that has wheels that only spin in one direction--forward. I know there is this device that I can attach to the axle or wheel to allow for the shaft to only spin in one direction and not spin in the other. What is this part called? (I need to know in order to be able to order it online)

Thanks.

...the thing that allows the axle or wheel to only spin "one way" is usually called a "one way" bearing.

They are designed so the bearing one goes in one direction and it locks up in the other.