A free-fall sensor is a component of electronic devices that detects them being in a state of free fall (after being dropped).
It is usually used to then apply safety measures, e.g. parking the head of a hard disk to prevent a head crash and resulting data loss upon impact. This device is included in the many common computer and consumer electronic products that are produced by a variety of manufacturers.
It is also used in some data loggers to monitor handling operations for shipping containers. The length of time in free fall is used to calculate the height of drop and to estimate the shock to the package.