antiparallel

Short definition from Wiktionary

Adjective

1. 1
   (of vectors) Parallel but in opposite directions: Describing the orientations of the two strands of DNA.
2. 2
   (of vectors) Parallel but in opposite directions: Describing two beta strands comprising a protein's secondary structure that are aligned and hydrogen bonded but where the vectors describing the amino- and carboxy-terminal ends of each strand are oriented 180° relative to one another.

Noun

1. 1
   A line that forms equal angles with two other lines, but in opposite directions.