¿Qué es Reduced-form model?

A reduced-form model of credit makes the default time $\tau_C$ of a name the first jump of a counting process with intensity $\lambda_t$ (a Poisson process with a time-dependent or random rate; a Cox process of One Quant Book 4, chapter 7, when the intensity is random), without modelling why the firm defaults: $\P(\tau_C\in[t,t+dt)\mid\tau_C>t) = \lambda_t\,dt$ and $Q_C(t) = \E[e^{-\int_0^t\lambda_s\,ds}]$.