a space-group symbol
/ Hermann-Mauguin -> HAIR-mahn moh-GAN /
A space-group symbol is a short code — like a compact number plate — that packs a crystal's whole symmetry into a few characters. The standard form is the Hermann-Mauguin (international) symbol: a single capital letter for the lattice centring, followed by the symmetry elements found along the crystal's special directions. Examples you will meet again and again are Fm-3m, P6_3/mmc, and Pnma.
Read it left to right. The first letter is the lattice centring: P (primitive), I (body-centred), F (face-centred), A or B or C (one-face-centred), R (rhombohedral). After that come one, two or three positions, one per symmetry direction of the crystal system. In cubic Fm-3m: F centring; then m along the cube edges (100 directions), -3 (a three-fold rotoinversion, read 'three-bar') along the body diagonals (111), and m along the face diagonals (110). In P6_3/mmc: P primitive, a 6_3 screw axis along c with a mirror perpendicular to it (the /m), then mirrors and a c-glide along the other two directions. In Pnma: P primitive, then glide/mirror planes n, m, a perpendicular to the a, b, c axes.
The symbol is a working tool, not just a label. Strip out the translation parts (turn every screw n_m back into a plain n, every glide letter back into m) and you can read off the point group and the crystal system; keep them and you can predict the systematic absences. There is an older Schoenflies form too — for instance D_4h^7 — but the Hermann-Mauguin symbol is the international standard because it directly displays the elements and their directions.
In Pnma the four characters are, in order: P (primitive lattice), then the planes perpendicular to a, b, c — an n-glide, a mirror m, and an a-glide. Just from the symbol you know it is orthorhombic, centrosymmetric, and nonsymmorphic (because of the n and a glides).
Each character after the centring letter belongs to one symmetry direction of the crystal system.
The full Hermann-Mauguin symbol and a short version both exist; software may print either. The three-digit space-group NUMBER (1 to 230) from the International Tables is unambiguous and often quoted alongside the symbol to avoid confusion.