A thermometer is a device that measures the temperature of things. The name is made up of two smaller words: Thermo means heat and meter means to measure. You can use a thermometer to tell the temperature outside or inside your house, inside your oven, even the temperature of your body if you're sick. thermometer is a device that measures the temperature of things. The name is made up of two smaller words: Thermo means heat and meter means to measure. You can use a thermometer to tell the temperature outside or inside your house, inside your oven, even the temperature of your body if you're sick.