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.
3. A flat domain is one where all elements except bottom are incomparable (equally well defined). E.g. the integers.
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Nearby terms: Flash Lights Impressively « flash memory « Flash ROM « flat » flat address space » flat ASCII » flat file