Even though the term co-crystal remains topic of debate, general consensus evolves towards defining co-crystals as crystalline compounds constructed of 2 or more neutral components which are, in their pure form, solid at room temperature and under atmospheric pressure. In most cases this definition clearly distinguishes co-crystals from salts or solvates. For solvates one of the components in its pure form, the solvent, is a liquid. A co-crystal is different from a salt since it is constructed from two distinct neutral components without any charge transfer taking place between components. Co-crystals can therefore be formed of components that lack ionizable functional groups.
Leyssens, T., & ter Horst Joop H. (2017). Solution co-crystallisation and its applications. In Multi-Component Crystals (p. p. 205-236). Walter de Gruyter GmbH. https://hdl.handle.net/2078.5/220463