Torricellian vacuum
Concise Oxford English Dictionary © 2008 Oxford University Press:
Torricellian vacuum /ˌtɔːrɪˈtʃɛlɪən, -ˈsɛlɪən/
▶noun a vacuum formed above the mercury column in a barometer when the tube is longer than the height of mercury sustainable by atmospheric pressure.
– origin from the name of the 17th-cent. Italian mathematician and physicist Evangelista Torricelli.
'Torricellian vacuum' also found in these Oxford entries:

