Cmme is number 67 of the 230 space groups: a C-centred orthorhombic lattice carrying the point-group class mmm. A general position in its cell is repeated 16 times.
It holds an improper operation, so a single enantiomer cannot crystallise in it. It contains the inversion, which halves the number of independent reflections and settles the phase problem differently from a non-centrosymmetric group. It is not symmorphic: no choice of origin removes its glide planes and screw axes, so systematic absences give it away in the diffraction pattern.
A diffraction pattern from a crystal in it shows the Laue symmetry mmm, so that is the most the symmetry of the intensities can tell you before the structure is solved.
Older literature spells it C m m a; the International Tables renamed it in 2002, and both names mean this group.
Molecular point groups with their operations and character tables, the 230 space groups with a cell builder, and the 17 wallpaper and 7 frieze groups drawn live. The tool needs JavaScript, and WebGL2 for the 3D views; these are the pages it offers: