An appliance that clips to the side of a pot or tub, heats the water and circulates it with a pump while a thermostat holds the temperature to within a fraction of a degree. Food sealed in a vacuum or zip bag is lowered into the bath and cooks through to exactly that temperature and no further: a steak held at 55 C is medium-rare edge to edge, an egg at 63 C is set in the white and liquid in the yolk, and nothing overcooks if left an extra half hour. Also called an immersion circulator or simply a sous vide machine; sous vide is French for "under vacuum".
Meat is usually finished with a hard sear for crust and colour, since the bath browns nothing. Long cooks at low temperature turn tough cuts tender while keeping them pink. Food safety depends on time and temperature together, so poultry and pork follow published tables rather than guesswork.
See sous vide for the cooking method and its time-and-temperature considerations.