  We use Lurie's symmetric monoidal envelope functor to give two new
descriptions of $\infty$-operads: as certain symmetric monoidal
$\infty$-categories whose underlying symmetric monoidal $\infty$-groupoids are
free, and as certain symmetric monoidal $\infty$-categories equipped with a
symmetric monoidal functor to finite sets (with disjoint union as tensor
product). The latter leads to a third description of $\infty$-operads, as a
localization of a presheaf $\infty$-category, and we use this to give a simple
proof of the equivalence between Lurie's and Barwick's models for
$\infty$-operads.
