In the music and technology worlds, “parallel” refers to things happening simultaneously.
Computing: two or more functions, operations, or processes occuring at the same time.
Electronics: a signal is split and fed simultaneously through two or more components. (this is the oppposite of “series,” where the signal feeds through two or more components sequentially.)
Music: a sequence of notes that are all harmonized by intervals of the same size (e.g., parallel thirds, parallel fifths).