The N connector is a threaded, weatherproof, medium-size RF connector used to join coaxial cables. It was invented in the 1940s by Paul Neill of Bell Labs, after whom the initial was chosen (although it is often claimed incorrectly that "N" is for "Navy").
The initial design was a 50 Ω connector for military systems operating below 4 GHz. In the 1960s, improvements pushed performance to 12 GHz and later to 18 GHz. 75 Ω variants exist (using a smaller center conductor diameter) but are rare (e.g. on some spectrum analyzers) and not compatible with 50 Ω connectors.
The N connector is seen as the input connector for Tektronix spectrum analyzers 1L20, 1L30, 1L40, 492, 2711, 2712, the 7250 digitizer, the output on the 108 pulse generator, input on the OT501/OT502/OT503/OT504, output on the OR502, the 2706 and 2707, and others.