1. [common] Lacking any complex internal structure. “That bitty box has only a flat filesystem, not a hierarchical one.” The verb form is flatten.
2. Said of a memory architecture (like that of the VAX or 680x0) that is one big linear address space (typically with each possible value of a processor register corresponding to a unique core address), as opposed to a segmented architecture (like that of the 80x86) in which addresses are composed from a base-register/offset pair (segmented designs are generally considered cretinous).
Note that sense 1 (at least with respect to filesystems) is usually used pejoratively, while sense 2 is a Good Thing.