 The constructible hierarchy L is likewise defined by recursion on ordinals. But whereas the full power set operation is iterated to obtain the cumulative hierarchy, the levels of the constructible hierarchy are defined strictly predicatively, that is by including at the next level only those sets which are first order definable using parameters from the previous level. More exactly, let Def(A) denote the set of all subsets of A definable in the structure &lt; A, ∈ &gt; by first order formulas with parameters in A. (For more on definability see the entry on  model theory  in this encyclopedia.)