A Wigner crystal is matter in a phase such that the particles form repeating patterns (as in other crystals) but without any specific bonds between them, forming the pattern merely because they are contained but repel each other. In astrophysics, it is thought that such patterns might occur within (some) white dwarfs, contributing to their equation of state, where gravity contains them and the significant repulsion is (I think) electron degeneracy. The term gravity crystal is sometimes used for Wigner crystals, considering their containment to be due to gravity or something analogous.
The Wigner crystal concept is a model which such real-world situations merely approximate. An ideal thought-experiment demonstrating the concept is electrons contained and cooled sufficiently that their mutual electrical repulsion holds them where they are, in which case they fall into a pattern that allows them to stay as far apart as they are allowed, such that the potential energy from their repulsion balances against their kinetic energy. The pattern that would result has been worked out, as well as the pattern that would result if the particles were confined to a plane, i.e., a 2-dimensional Wigner crystal. Also, for a 1-dimensional Wigner crystal, which would merely space the particles evenly in a row. The concepts can be demonstrated using normal-life sized particles in one and two dimensions, and the one-dimensional case has been demonstrated at an atomic level. All three crystal dimensions have been simulated by computer.