We investigate the expressive power and computational complexity of [Escr ][Lscr ]^ν, the extension of the lightweight description logic [Escr ][Lscr ] with concept constructors for greatest fixpoints. It is shown that [Escr ][Lscr ]^ν has the same expressive power as [Escr ][Lscr ] extended with simulation quantifiers and that it can be characterized as a largest fragment of monadic second-order logic that is preserved under simulations and has finite minimal models. As in basic [Escr ][Lscr ], all standard reasoning problems for general TBoxes can be solved in polynomial time. [Escr ][Lscr ]^ν has a range of very desirable properties that [Escr ][Lscr ] itself is lacking. Firstly, least common subsumers w.r.t. general TBoxes as well as most specific concepts always exist and can be computed in polynomial time. Secondly, [Escr ][Lscr ]^ν shares with [Escr ][Lscr ] the Craig interpolation property and the Beth definability property, but in contrast to [Escr ][Lscr ] allows the computation of interpolants and explicit concept definitions in polynomial time.