A sequence of nucleic acid
) that has been evolved
to bind to another molecule
Aptamers could theoretically be used as some kind of pharmaceutical product, but they are generally made for experiments to verify molecular evolution hypothesis. For example, it has been proposed that there is a chemical relationship between amino acids and codons. This would suggest that the codons were the binding regions of aptamers that evolved naturally.
Of course, binding is one step towards catalysis. If you can make RNA bind a small molecule, you can evolve it to catalyse some reaction with that molecule. These are then ribozymes, or deoxyribozymes, depending on the nucleotide.