  We derive an integral condition for core-collapse supernova (CCSN) explosions
and use it to construct a new diagnostic of explodability. The fundamental
challenge in CCSN theory is to explain how a stalled accretion shock revives to
explode a star. In this manuscript, we assume that the shock revival is
initiated by the delayed-neutrino mechanism and derive an integral condition
for spherically symmetric shock expansion, $v_s > 0$. One of the most useful
one-dimensional explosion conditions is the neutrino luminosity and
mass-accretion rate ($L_{\nu}-\dot{\mathcal{M}}$) critical curve. Below this
curve, steady-state stalled solutions exist, but above this curve, there are no
stalled solutions. Burrows & Goshy suggested that the solutions above this
curve are dynamic and explosive. In this manuscript, we take one step closer to
proving this supposition; we show that all steady solutions above this curve
have $v_s > 0$. Assuming that these steady $v_s > 0$ solutions correspond to
explosion, we present a new dimensionless integral condition for explosion,
$\Psi > 0$. $\Psi$ roughly describes the balance between pressure and gravity,
and we show that this parameter is equivalent to the $\tau$ condition used to
infer the $L_{\nu}-\dot{\mathcal{M}}$ critical curve. The illuminating
difference is that there is a direct relationship between $\Psi$ and $v_s$.
Below the critical curve, $\Psi$ may be negative, positive, and zero, which
corresponds to receding, expanding, and stalled-shock solutions. At the
critical curve, the minimum $\Psi$ solution is zero; above the critical curve,
$\Psi_{\rm min} > 0$, and all steady solutions have $v_s > 0$. Using
one-dimensional simulations, we confirm our primary assumptions and verify that
$\Psi_{\rm min} > 0$ is a reliable and accurate explosion diagnostic.
