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 - a breathing device that is placed over the mouth and nose; supplies oxygen from an attached storage tank. 1  aqualung, Aqua-Lung, scuba - a device (trade name Aqua-Lung) that lets divers breathe under water; scuba is an acronym for self-contained underwater breathing apparatus.