acorn valve
or acorn tube
n (Electronics) a small electronic valve, approximately acorn-shaped with small closely-spaced electrodes, used in ultrahigh-frequency applications
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014