SMA connectors are miniature coaxial 50 Ω connectors good to about 20 GHz.
The male connectors have a protruding center pin and ground shroud that screws on. The female is threaded on the outside. (Note that in the "Reverse Polarity" or "RP-SMA" connectors found in WLAN equipment, the pin is swapped to the shell with the exterior thread.)
SMA are somewhat similar to UHF connectors, but much smaller - initial Tektronix literature(Note1) calls them "3 mm connectors" - and with much tighter tolerances and better controlled impedance. SMA connnectors are used on the S-50, S-51, S-52, 7T11, S-4, S-6 and many later GHz-class instruments.
(1) See Service Scope No. 52, Oct 1968 p.16, Something New in Oscilloscope Connectors