SELEX is an acyronym for "systematic enrichment of ligands by exponential enrichment, the experimental process by which large libraries (ca. 10¹⁴) of unique single-stranded RNA or DNA sequences are subjected to iterative cycles of screening, selection, and amplification; the process results in the isolation of sequences in the library that perform a desired task, for example, high-affinity binders to a certain protein."¹

1. Feldheim, D. L.; Eaton, B. A. Selection of Biomolecules Capable of Mediating the Formation of Nanocrystals.  ACS Nano 2007, 1, 154–159.