The evacuation tip is a port on a nixie

Nixie tube

A nixie tube is an electronic device for displaying numerals or other information. The glass tube contains a wire-mesh anode and multiple cathodes. In most tubes, the cathodes are shaped like numerals. Applying power to one cathode surrounds it with an orange glow discharge...

 or vacuum tube

Vacuum tube

In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 used to evacuate gases from the tube, and in the case of nixies, add the proper mix of gases before being sealed. It provides a convenient port for the amateur glasshacker to introduce gases to alter the performance of the tube. It is also known as the "tubulation" or "pip".