That is a movable (less fixed) connector of a mating pair, which shall be designated "P" where P means plug. This convention is currently defined in ASME Y14.44-2008 which is the current actively maintained follow on to the withdrawn IEEE 200-1975; IEEE 200-1975 was derived from the long withdrawn MIL-STD-16 (which dates back at least to the 1950s which highlights the heritage of this connector naming convention). IEEE 315-1975 works alongside ASME Y14.44-2008 to define J, P and X references.
That is a movable (less fixed) connector of a mating pair, which shall be designated "P" where P means plug. This convention is currently defined in ASME Y14.44-2008 which is the current actively maintained follow on to the withdrawn IEEE 200-1975; IEEE 200-1975 was derived from the long withdrawn MIL-STD-16 (which dates back at least to the 1950s which highlights the heritage of this connector naming convention). IEEE 315-1975 works alongside ASME Y14.44-2008 to define J, P and X references.
Nice job. Looks like a sleeved 3 pin fan connector or just a cable extender. But odd looking for sure. Looks like a coolant cord though.3 conductor fan cable from a ATX mother board. Most probably the male end of an extension cable for same. Less likely a proprietary ATX power supply fan monitor cable for reporting power cooling fan RPMs.
So I'm an old geek and I build PCs.