In pro audio gear, a metal, plastic, or cloth mesh that is stretched or mounted in front of a speaker driver or a microphone diaphragm. A grille is intended to protect the driver or diaphragm from damage. It must be as acoustically transparent as possible so as not to interfere with the accurate reproduction or capture of sound waves.
Don’t confuse this with a “grill,” which is a metal grate used for cooking food over an open fire, the process of cooking food over an open fire, a griddle, a device for cooking outdoors, or the act of interrogating someone. Trying to grill with a grille would likely be catastrophic, at least for the food you’re attempting to cook.
While grills may be useful studio or live sound accessories, grilles are even more essential pro audio devices.