Wiktionary
crystal twinning
n. The intergrowth of two twin crystals.
Wikipedia
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.
Twinning can often be a problem in X-ray crystallography, as a twinned crystal does not produce a simple diffraction pattern.