To be officially in correction (down 10% or more), the closes are used.

The highest close in the S&P 500 (^GSPC) was 2930.75 on September 20, 2018.

Multiply by 0.9, round down, and we get the magic level at which the S&P 500 must close to be in correction:

2637.67

The market breached this level today, so that has generated some “S&P 500 in correction” headlines. But it’s not official until the close.

Also, markets do not fall in and out of correction. Once a market enters correction, it stays there until it:

1) Rallies to a new high (most accepted definition)

2) Rallies 10% on a close-to-close basis (another accepted definition)

3) Enters a bear market (down 20% close to close)