1. Lacking any complex internal structure. "That bitty box has only a flat file system, not a hierarchical one." The verb form is flatten. Usually used pejoratively (at least with respect to file systems).
2. Said of a memory architecture like that of the VAX or Motorola 680x0 that is one big linear address space (typically with each possible value of a processor register corresponding to a unique address). This is a Good Thing. The opposite is a "segmented" architecture like that of the Intel 80x86 in which addresses are composed from a base-register/offset pair. Segmented designs are generally considered cretinous.