Gigai

A is an artificial body that allows Shinigami to remain in the Human World and interact with humans.

Overview
A Gigai can for an extended period, either due to temporary loss of powers or extended missions. In certain circumstances, a Shinigami will need a way to stay in the Human World for long periods of time, interact with the living, and/or recover their powers when the need arises, all of which they can't do in spirit form. Thus, the Gigai was developed. While the body is synthetic, it functions and operates exactly like a natural one, including but not limited to blood flow and the like. Unlike the normal Shinigami body, a Gigai is detectable by humans who are not at all spiritually aware, thereby allowing the Shinigami to interact with them. This also means that a Shinigami in a Gigai has to behave like a normal Human, this task can prove to be somewhat challenging to inexperienced Shinigami, due to the cultural and technical differences between the Human World and Soul Society.

Variations
Kisuke Urahara experimented with Gigai at length, and created at least two variations. The most notable one is an untraceable Gigai which drains the user's spiritual power instead of restoring it, effectively making them Human if used long enough. He also developed an inflatable Gigai for use as a decoy in combat.