Chemists long assumed that all matter existed in one of three states (gaseous, liquid, or solid) depending on the temperature and pressure. However, during the nineteenth century it was recognized that many chemical systems did not fall into these categories. For example, fine clay suspensions can be considered neither true solids nor true solutions. Gels and smoke are other examples of states of matter that are difficult to define. Since many of these apparently anomalous systems are amorphous, they were called colloid, which means glue.

As knowledge of these systems progressed, they were divided into two categories: hydrophilic (water-loving) colloids, which are stabilized by strong solute-solvent interactions; and hydrophobic (water-hating) colloids, which are stabilized kinetically. Polymer solutions and gels are typical examples of the first category, and hydrosols of the second. While these terms are still used, they are not representative of the large number of systems now considered colloids. This is particularly true for self-assembly systems, recognized as part of the colloid domain in the midtwentieth century only. Furthermore, these terms can in some cases be misleading since, for example, the so-called hydrophobic colloids are not really hydrophobic.