  In this paper, we study different notions of solutions of nonlocal and
nonlinear equations of fractional $p$-Laplace type $${\rm P.V.} \int_{\mathbb
R^n}\frac{|u(x)-u(y)|^{p-2}(u(x)-u(y))}{|x-y|^{n+sp}}\,dy = 0.$$ Solutions are
defined via integration by parts with test functions, as viscosity solutions or
via comparison. Our main result states that for bounded solutions, the three
different notions coincide.
