RS-232 A serial interface published by the EIA for asynchronous data communication over distances up to a few hundred feet. Characterized by a single-ended (not differential) physical layer, it uses one signal wire for transmission, another for reception, and a common wire (ground), plus some timing and control signals. Its specifications are rooted in electromechanical equipment signaling (Teletype machines). Still a very common interface but largely replaced by USB in recent years.
The term “serial” interface is often used for an RS-232 interface. The usage is not quite accurate — while RS-232 is a serial interface, there are other serial interfaces in addition to RS-232.
When it was introduced in 1987, rapidly became the most common way to implement RS-232 because it required only a single 5-volt supply. On-board DC-DC converters developed the odd voltages required by the official spec.