The repetition of syntactical similarities in passages closely connected for rhetorical effect, as in Pope's An Epistle to Dr. Arbuthnot:
Happy my studies, when by these approved!
Happier their author, when by these beloved!
The repetitive structure, which is commonly used in elevated prose as well as poetry, lends wit or emphasis to the meanings of the separate clauses, thus being particularly effective in antithesis.
Sidelight: Sometimes the use of parallel structures is extended throughout an entire poem.
(Compare Anadiplosis, Anaphora, Echo, Epistrophe, Epizeuxis,
Incremental Repetition, Polysyndeton, Refrain, Stornello Verses)

This definition is in context to Poetry. See more contextual defintions for Parallelism.