Cryostasis

Cryostasis is a preservation method where a human is preserved through lowering of their body temperature. In the Metroid universe, this technology appears to be used for the purpose of preserving the pilot or crew of a spaceship when travelling through space for long periods of time. It is only briefly mentioned at the beginning of Metroid Prime 3: Corruption, when Samus Aran is awoken from cryostasis by the Chozo biotech computer within her Gunship. Samus lies in her reclined pilot seat while in stasis, which automatically rises as she awakens. Her Varia Suit is then activated, and the mission begins.

Although not observed in other games, it is possible that Samus uses cryostasis when travelling between worlds during her other missions. However, in Metroid: Other M she is seen piloting her Gunship through space for weeks before receiving the Baby's Cry signal that leads her to the BOTTLE SHIP, and afterward when returning to Galactic Federation Headquarters.

Trivia

 * Cryostasis is more prevalent in the Alien film franchise, which greatly inspired the Metroid series. It is called Hypersleep and is always used when travelling through space. Hypersleep is presented as taxing on the body as some characters suffer from respiratory problems, while Samus has no visible trouble upon awakening from cryostasis.
 * In the Metroid 1.5 design document, which eventually became Metroid Prime 2: Echoes, Samus was to enter her Cryo chamber for a rest after her mission on Tallon IV, while her ship travelled home via autopilot. She would have been awakened upon landing on the large warship that was the setting of the design pitch.