Refers to things being arranged in a series, or one item after another. In computers and data, serial is a sequential transmission of each piece of information. Literally this means each bit, of a byte, one at a time over one wire. Parallel transmission uses many wires to simultaneously transmit large packets of data. Many types of computer data are transmitted serially. This is how modems and fax machines communicate, for example. In music MIDI data is transmitted serially both in the MIDI cables themselves and into and out of the computer via a MIDI interface. In order to transmit more data per second in a serial system one must speed up the rate of transfer. Since all MIDI is standardized to a set speed there is a very finite amount of MIDI data that can be transmitted on one cable. It is possible, however, for a MIDI Interface to be connected to a host computer at a high enough transfer rate to be able to handle many individual MIDI streams “simultaneously” (though we don’t mean that literally, it is still serial).