A Dimensiometer is a device that enables the possessor to shift their body into a two-dimensional, intangible "shadow" state.
While the device is activated, the possessor can move quickly and silently across and through most surfaces and materials, all the while remaining impervious to physical contact and attack. Long-term side effects from prolonged use of the device are unknown, however, it seems to provide the possessor with access to the Shadowlands.
Shadow Thief (Carl Sands) was given a dimensiometer Shadow Field belt by the Thanagarian criminal Byth to assist in his thefts and schemes against Hawkman. Sands later lost the belt to the Phantom Stranger. Years later, the belt reappeared in the possession of Cassandra Cain, who was using it in her guise as Kasumi while working with the Justice League Elite.
After the Infinite Crisis, Booster Gold took possession of the Dimensiometer, among other devices, in an effort to give Rip Hunter enough tools to create something to fight Skeets; who was being controlled by Mister Mind to alternate the Multiverse.