Displacement
  Effective address is found by adding a constant value specified as one of
    the operands to the address stored in a named register(indirect).

  Value is usually unsigned.

  Range is usually less than possible address range.
    IBM 360: L R1,23(R4,R2) - 12 bit (<4096) displacement
 
  Usually part of a more complex addressing mode.
    Above example is register indirect(base), indexed, with displacement.