Numitron is a trademark of the RCA corporation, the name of a type of display tube. A Numitron tube, also sometimes known as a minitron, contains wire filaments stretched across the areas to be illuminated, where the LEDs would usually be mounted in a modern 7 segment display.
The Numitron tube illuminates the digit segments by allowing current to heat the filaments to a dull orange or white glow.
A Numitron's package is a glass tube, similar to that of a Nixie tube or small valve. The air is removed from the tube, preventing the filaments from burning out (which they eventually would after some time in service).
Numitron tubes may still be found in some old gas pumps - there's a gas station down the road from me here which uses that type of display on their pumps, which have been partly modernized - they're equipped with 1970s display technology, and yet, paradoxically, allow the customer to pay at the pump using a credit card.