D-lock 
noun
A mechanism used to secure a bicycle or motorbike when parked, consisting of a U-shaped bar which is locked to a crosspiece of solid metal.It is best to lock them in a garage or shed or if you are going to leave them out in a garden secure them with a D-lock....
- When you're out and about, shackle locks, also known as D-locks, are good for fixing your moped to street furniture, such as lamp posts.
- The blockade caused chaos on the A59 as protesters, using D-locks, tripods and chains, closed and bolted the two side entrance gates.