To be specific, the spring equinox was at 0514 UTC (or 1:14 a.m. EDT) this morning. Some have begun calling this date the March equinox and the autumnal equinox the September equinox to avoid assumptions that Spring is in March and autumn is in September. That's true for those in the Northern Hemisphere, but exactly the opposite occurs in the Southern Hemisphere.
When the spring or vernal equinox (or March equinox) arrives, it signifies that the sun crosses the celestial equator as it travels north. Although not specifically true, for the most part the spring equinox signifies that across the globe today, there are approximately equal amounts of daylight and sunlight. In reality, however, the actual dates of such an occurrence depends on how far north or south a location is.
The autumnal equinox occurs with a similar situation in the autumn, or fall. In 2012, it will occur on Sept. 22, at 1449 UTC.
The Vernal equinox and autumnal equinox names are classical names which are direct derivatives of the Latin (ver = spring and autumnus = autumn). The word “equinox” also derives from Latin. It would mean“equal night" as translated, though as noted, the exact date when an equinox occurs depends on a person's location.
For example, this year, Washington, D.C. saw the day when day equaled night in terms of length occur on March 16. Sunrise on March 16, 2012 was 7:17 a.m. EDT and sunset was at 7:17 p.m. EDT.
Some are saying the March equinox is coming a day earlier this year because this is also a Leap Year. There is some truth to that, as without the Leap Year the equinox would occur on March 21. However, this trend has run true since 2008. As noted by the United States Navy Observatory Portal, from 2008 to 2017, March 20 has and will continue to be the day of the March equinox. Still, given the timing of the equinox, this is the earliest equinox since 1896.