Word of the Day is designed to help children learn how to use a dictionary, whilst simultaneously expanding their vocabulary. Today’s Word of the Day begins with the letter ‘T’. The word is: Thermal.thermal adj. Of heat; determined, measured, operated by heat; ~ capacity, (of a body) number of heat units required to raise its temperature by one degree; ~ springs, hot springs; ~ unit, unit of heat; British T~ Unit, (abbrev. Btu) amount of heat required to raise the temperature of one pound of water at its maximum density by one degree Fahrenheit. ~ n. Rising current of warm air.