If you have seen the abbreviation BCT somewhere related to materials science, physics, or crystallography, it probably stands for Body-Centered Tetragonal. This is a crystal structure with 8 atoms forming the corners of a tetragon (elongated or shortened cube), and one atom in the center.
There is both a BCT Bravais lattice, and BCT crystal structure. The BCT Bravais lattice is a general, mathematical description of lattice point symmetry. The BCT crystal structure is when each lattice point exactly corresponds to an atom.
Indium, protactinium, and beta-mercury exhibit the BCT crystal structure, while martensite in steel has a more complex atomic arrangement this isn’t really a BCT crystal structure, but may still be called BCT because it fits in the BCT Bravais lattice.
For more information on Bravais lattice or crystal structures, you can read this article.
You may also be interested in some related terms.