Parallel could mean:

• Parallel (geometry), as in "parallel lines"
• Circle of latitude, an imaginary east-west line circling a globe, e.g. the 38th parallel
• Parallelism (grammar), a balance of two or more similar words, phrases, or clauses
• Parallel study, a type of clinical study
• Parallel circuits, as opposed to series
• Parallel import, also known as a "grey market product"
• Parallel transport, in mathematics using vectors and smooth curves

In music:

• Parallel key, the minor (or major) key of a major (or minor) key with the same tonic
• Harmonic parallelism, also known as "harmonic planing" or "parallel voice leading"
• Parallel fifths, also known as "consecutive fifths", involve the concurrence of successive intervals of a perfect fifth between two voices in parallel motion -- a harmonization generally forbidden in modern musical theory

Other:

• Parallel (manga) (ぱられる) is a shōnen manga by Toshihiko Kobayashi
• Parallel 9, A British children's television show