I think think that's kind of a philosophical question. Wikipedia, for example, explains path tracing as integrating over incident light on a surface. You could either think of that as incident light at a point on a virtual film/sensor plane behind a lens or incident light on the first object you hit in the scene.
Such techniques are often generally known as "Global Illumination".
Often in physics based rendering you are attempting to estimate the BSDF (bi-directional scattering distribution function, often broken into T, transmission and R reflection) parts. This gets complicated by interactive media and subsurface scattering (i.e the photons go into a material and bounce around) but in the simple case of a surface interaction you are estimating an integral over solid angle for all light interactions. Monte Carlo path tracing is one way to do this estimation.
