1. Lacking any complex internal structure. "ThATbitty 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 flATdomain is one where all elements except bottom are incomparable (equally well defined). E.g. the integers. [Jargon File]