A stress fracture is one type of incomplete fracture in bones. It is caused by unusual or repeated stress." This is in contrast to other types of fractures, which are usually characterized by a solitary, severe impact. It could be described as a very small sliver or crack in the bone; this is why it is sometimes dubbed "hairline fracture". It typically occurs in weight-bearing bones, such as the metatarsals(bones of the foot).
It is a common sports injury, and most cases are associated with athletics.