What are Orthogonal DNA Sequences?
Orthogonal DNA sequences are the shortest possible single stranded nucleotide stretches constructed. These sequences are used as probes in DNA hybridization. Because of its length and construction it has least chance of binding with other probes. 

This technique improves the efficacy and specificity and is clinically used to detect genetic disorders in shortest time duration.

Via:
https://www.nature.com/articles/srep24650

https://www.pnas.org/content/106/7/2289