The word equinox comes from the Latin words for "equal night." Around the time of the fall and spring equinoxes, there are equal days of daylight and darkness.
However, if you look at your Sun rise and set times, you'll notice that there aren't exactly 12 hours of light and dark. Why not?
On the equinoxes, the very center of the Sun sets just 12 hours after it rises. But the day begins when the upper edge of the Sun reaches the horizon (which happens a bit before the center rises), and it doesn't end until the entire Sun has set.
Not only that, but the Sun is actually visible when it is below the horizon, as Earth's atmosphere refracts the Sun's rays and bends them in an arc over the horizon.
According to our former astronomer, George Greenstein, "If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have 'equal nights.'"