synthon

This term is a highly specialized technical expression used almost exclusively within the field of organic chemistry, specifically in retrosynthetic analysis. It describes an idealized fragment that represents a structural unit, often carrying a theoretical charge, which guides the chemist in selecting actual chemical reagents (synthetic equivalents) to build a target molecule.

Because it refers to a conceptual building block rather than a physical substance you can hold in a flask, it is distinct from a reagent. While a reagent is a tangible chemical, a synthon is a theoretical tool used for planning the architecture of a synthesis.