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 Motorola680x0 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. 3. A flat domain is one where all elements except bottom are incomparable (equally well defined). E.g. the integers. [Jargon File]