A Basal thermometer is an ultra sensitive thermometer that measures even the smallest change in your body (basal) temperature.
Basal Temperature is the normal body temperature of a healthy person right after waking in the morning. In women the basal temperature normally rises just after ovulation because of hormonal changes.
You can use a Basal Thermometer to measure this small increase in temperature near the middle of the menstrual cycle to help tell you when ovulation has occurred.
There are different types of Basal Thermometer, but the best have a digital display, so they are very easy to use and read. A Basal thermometer will often come with a blank Basal Temperature chart for you to record and chart chages in your Basal temperature.