  We introduce the boundary effect on the ground state as an attribute of
general local spin systems that restricts the correlations in the ground state.
To this end, we introduce what we call a boundary effect function, which
characterises not only the boundary effect, but also the thermodynamic limit of
the ground state. We prove various aspects of the boundary effect function to
unfold its relationship to other attributes of the system such as a finite
spectral gap above the ground state, two-point correlation functions, and
entanglement entropies. In particular, it is proven that an exponentially
decaying boundary effect function implies the exponential clustering of
two-point correlation functions in arbitrary spatial dimension, the
entanglement area law in one dimension, and the logarithmically corrected area
law in higher dimension. It is also proven that gapped local spin systems with
nondegenerate ground states ordinarily fall into that class. In one dimension,
the area law can also result from a moderately decaying boundary effect
function, in which case the system is thermodynamically gapless.
