The term "liquid crystal" seems, at first glance, to be a contradiction, as we are accustomed to equating crystals with solids. There are some important differences between solids and crystals, however. In a crystal, molecules are ordered, but not necessarily held rigidly in place. In contrast, in solids the orientation and positions of the atoms, ions or molecules are fixed, but not necessarily ordered with respect to each other. Often, both of these properties coincide in the same material to give rise to a crystalline solid. However, this isn't always the case. Many solids are highly disordered, with the constituent parts frozen inplace in a disorganized fashion. These "amorphous solids" are really quite common, especially in the case of polymers. Plastic bottles, wooden tables and glass windows are all examples of solids that are disordered at the molecular level.
Just as solids can be disordered, it is possible for liquids to be ordered. Although we are more familiar with isotropic liquids, in which the constituent molecules are completely disordered (left), it is possible to have anisotropic liquids that exhibit varying degrees of positional and orientational ordering at the molecular level. These are liquid crystals.
There are a variety of ways in which liquid crystals can be classified: according to the ordering of the molecules within the phase, by the shapes of constituent molecules, or according to the conditions under which the l.c. phase is formed. We will introduce these concepts in the following sections, but it is important to bear in mind that these terms describe different aspects of liquid crystallinity and are therefore neither mutually exclusive or inclusive. A common mistake, for example, is to use the terms "discotic" and "columnar" interchangeably: discotic molecules may also form nematic phases, and many nondisc-shaped molecules exhibit columnar ordering.
Before continuing, there are some terms that are commonly used in the field of liquid crystals. Since liquid crystal phases are intermediate between crystalline solids and isoptropic liquids, they are often referred to as "mesophases" ("meso" is Latin for middle). Likewise, molecules that form mesophases are referred to as "mesogens" or "mesogenic."