thermometer

Concise Oxford English Dictionary © 2008 Oxford University Press:
thermometer/θəˈmɒmɪtə(r)/

▶noun an instrument for measuring and indicating temperature, typically consisting of a graduated glass tube containing mercury or alcohol which expands when heated.

– derivatives
thermometric adjective,
thermometrical adjective,
thermometry noun.

– origin C17: from Fr. thermomètre or mod. L. thermometrum, from thermo- + -metrum ‘measure’.