A fixed-gear bicycle, often called a "fixie," is a type of bike that has a single gear and no freewheel mechanism. This means that the pedals are always in motion when the bike is moving, allowing the rider to control speed and braking by pedaling backward. Fixed-gear bikes are popular for their simplicity and lightweight design. These bicycles are commonly used in urban environments and by cycling enthusiasts. They require less maintenance than multi-gear bikes and offer a direct connection between the rider and the bike. However, they can be challenging to ride, especially on hilly terrain, due to the lack of gear options.