A slow cooker
How Much Time You Can Safely Keep Your Slow Cooker Set On 'Warm'
The "warm" setting on a slow cooker keeps food at the ideal serving temperature until it's time to eat, but there's a limit to how long food can be safely kept in this environment.
Per the USDA, food-borne bacteria grow at temperatures between 40 and 140 F. Most "warm" slow cooker settings keep food at around 145 F, which is quite close to this range.
You should only keep food on "warm" for up to four hours, and never use the warm setting to cook or reheat foods, as the temperature won't be hot enough to kill off bacteria.
Different slow cooker models have different settings, so consult your manual or use a thermometer to check the actual temperature of your food under the warm setting.
Lastly, know that every time you open the slow cooker's lid, the temperature of your food may drop by 10 to 15 F, so minimize opening the lid while your food is being kept warm.