What is a heifer cow? A heifer is a mature female cow that has not calved, or given birth to a calf, yet. As soon as a heifer gives birth, she becomes a cow. A cow refers to a female of any age that has given birth.
When most people think about gas masks or respirators, what they usually envision is a tight-fitting plastic or rubber face mask with some sort of filter cartridge. The mask covers the nose and mouth. These are called half-mask air-purifying respirators.
oxygen mask. n. A masklike device placed over the mouth and nose and through which oxygen is supplied from an attached storage tank. oxygen mask. n. (Medicine) a device, worn over the nose and mouth, to which oxygen is supplied from a cylinder or other source: used to aid breathing. oxâ²ygen mask`. n. a masklike device placed or worn over the nose and mouth when inhaling oxygen from an attached tank.