Circular connectors were invented in the early 1930s when Douglas Aircraft Company contracted ITT Interconnect Solutions (known then as Cannon) to develop connectors for their aircrafts. Circular connectors were born! Within the decade, they were produced in mass volumes and many were sent overseas to fight in WWII.
Widely used these days and easy to find, circular connectors are named after their cylindrical contact housings and circular contact interface geometries. They are generally easy to mate, can hold a range of contact types, and allow for different contact voltages and currents. Plus, they can be made extra tough for use in more rugged, dangerous environments, employing environmental sealing to keep out all dust and dirt. You can find circular connectors being used in a number of different industries including industrial work, aerospace, the medical field, and military connectors.
As far as design goes, many circular connectors utilize crimp contacts, which are pins or sockets whose back portion is a hollow cylinder. The stripped wire (the conductor) is inserted into this hollow and then the walls of the hollow are compressed until they have a firm hold on the wire.
Circular connectors are often mounted on plates called flanges that are attached to a panel with screws. The one major disadvantage to circular connectors is that when used in a panel, the circular design reduces the amount of usable space on the panel.
Along with d-sub, USB connectors, and power connectors, circular connectors are among the most common electrical connectors utilized today.