or spring washer
It's a type of lock washer
It's placed between the bolt head and what it is bolted to -theoretically locking the bolt in place by the tension it exerts.

I bring this up because I had a bad experience.
I replaced the noisy transmission bearings in Dreya's red Civic so they wouldn't explode.
Then I drove it 300 miles w/o issue, called Dreya and told her the car was done. The next day the shifter was slightly non compliant. I called Dreya back to tell her I had to keep the car until the weekend (another week) when I would have time to investigate and I parked the car.

Last Saturday I took the shifter linkage loose from the transmission to isolate it and shifted the transmission by hand... it seemed fine... at first. A little more wiggling around and I heard something fall down inside the transmission...tink-tink-tink-dink-dink and then suddenly the shifting was all wrong. Very very wrong.

So I had to take the transmission out and apart AGAIN.

I found that a bolt fell out. That's what I heard make the tink-tink-tink-dink-dink noise. The bolt and spring washer were found on the collector magnet inside the transmission.

It's possible I just didn't tighten the bolt when I had the transmission apart to do the bearings. The spring washer wasn't very springy though. This time I used thread locking compound on the bolt. I couldn't get a new spring washer.

That's a story of how I was very lucky.

So here's some pics after the jump... where the bolt was supposed to be, and where it ended up.