Emergency shelters were structures designed to provide refuge in the case of an emergency.
A number of these shelters were constructed by Starfleet in the 23rd century for use in drastic circumstances where housing large numbers of people was a priority. They were constructed of thermal concrete. (TOS: "The Devil in the Dark")
In 2372, when a fleet of Klingon starships surrounded Deep Space 9, Constable Odo told Quark that he should be in an emergency shelter, but Quark insisted on staying in his bar to defend it. (DS9: "The Way of the Warrior")