Wiktionary
n. A helmet intended to be worn while riding a bicycle.
Wikipedia
A bicycle helmet is designed to attenuate impacts to the head of a cyclist in falls while minimizing side effects such as interference with peripheral vision. There is ongoing scientific research into the degree of protection offered by bicycle helmets in the event of an accident, and on the effects of helmet wearing on cyclist and motor vehicle driver behaviour. There is active debate over what can be concluded from available studies, and on whether the use of helmets by cyclists should be promoted or mandated, either just for children, or for cyclists of all ages. In particular the debate over bicycle helmet laws is intense and occasionally bitter, often based not only on differing interpretations of the scientific and other academic literature, but also on differing assumptions and interests of various parties.
Usage examples of "bicycle helmet".
But the mid-thirties unit, seated under a blurry photo of the statue of the marine, hair to his shoulders, Nicaraguan peasant necklace, bicycle helmet on the table, now this was a GEE frogman.
I didn't know her, but I did recognize the bicycle helmet under her arm.
Outside, they watched a man in a tubular bicycle helmet ride past her car on his ten-speed, getting his morning exercise.