What is an expansion or protuberance on a stem or tube, as the bulb of a thermometer, which may be of any form, as spherical, cylindrical, curved, etc. Tomlinson called?

bulb — An expansion or protuberance on a stem or tube, as the bulb of a thermometer, which may be of any form, as spherical, cylindrical, curved, etc. Tomlinson. (Webster's 1913.)