bcc: body-centred cubic
Definition:The bcc system refers to a cubic unit cell with lattice points on its eight corners and one point in its centre.
Explanation:According to the point lattice model in crystallography, the atomic nuclei of a bcc lattice are located at the corners and the center of the unit cell. The diagonals through the center of the cell, <111>, correspond to the closest packed direction of the bcc lattice. Since the volume of the unit cell is not completely filled by the atomic spheres, interstitial sites exist between the atoms. These interstitial sites can be classified as either octahedral or tetrahedral depending on their geometry (Fig. 2).
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Fig. 1: (a) Unit cell of a bcc point lattice. (b) Unit cell according to the ball model. (c) Geometric relationships in the bcc lattice.

Fig. 2: Interstitial sites in the bcc lattice.
SFB-Link:The knowledge on the crystal structures of steels plays an important role in understanding of the mechanisms, which explains the sophiscated material behaviours of high Mn steels.
References:Materials Science of Steel – Textbook for Students at RWTH Aachen, W. Bleck (Ed.) 3rd edition 2013.