sentences:  The expression
  
  for "A" an "L"-structure (or "L"-model) in a language "L", "φ" an "L"-formula, and formula_2 a tuple of elements of the domain dom("A") of "A".In other words, formula_3 denotes a (monadic) property defined on dom(A).In general, where "x" is replaced by an "n"-tuple formula_4 of free variables, formula_5 denotes an "n"-ary relation defined on dom("A").