a body-centered lattice
Picture a box with a lattice point at each of its 8 corners and one more point smack in the middle of the box. That extra central point makes it a body-centered lattice (symbol I, from the German innenzentriert, meaning inner-centred). The centre point is a genuine lattice point — its surroundings are identical to a corner's.
Counting: 8 corners times 1/8 + 1 centre times 1 = 2 lattice points per cell, so the conventional body-centered cell is twice the primitive volume. The centre sits at fractional coordinate (1/2,1/2,1/2). For example, iron at room temperature (alpha-Fe, ferrite) is body-centered cubic — an Fe atom at (0,0,0) and one at (1/2,1/2,1/2), each with 8 nearest neighbours.
Do not confuse the body-centered lattice with the BCC structure. When the motif is a single atom, the two coincide; but body-centering is a lattice property (a centering type) available to several crystal systems (cubic I, tetragonal I). Its diffraction signature: reflections appear only when h+k+l is even.
Body-centered cubic tungsten: a = 3.165 angstrom, atoms at (0,0,0) and (1/2,1/2,1/2). A diffraction scan shows (110), (200), (211) peaks but no (100) or (111) — because h+k+l must be even. That missing (100) is the fingerprint of body centering.
In BCC, reflections with h+k+l odd are systematically absent.
Body-centered cubic packing is NOT close-packed: its packing factor is 0.68 (coordination number 8), less than the 0.74 of FCC. Body centering is about lattice points, not about maximum density.