symmetric monoidal (∞,1)-category of spectra
The notion of smooth $(\infty,1)$-algebra is the analog in higher category theory of smooth algebra. This is the basis for the derived geometry version of differential geometry/synthetic differential geometry.
A smooth $\infty$-algebra is an ∞-algebra over an (∞,1)-algebraic theory $T$ for $T$ the ordinary Lawvere theory of smooth algebras.
The model structure on simplicial algebras on simplicial C-∞-ring is a presentation for smooth $(\infty,1)$-algebras.
Smooth $(\infty,1)$-algebras appear as the algebras of functions in derived differential geometry, for instance on derived smooth manifolds.