ID is an Inside Day.

If the high of the current day is lower than the high of the previous day AND the low of the current day is higher than the low of the previous day then we have an ID or Inside Day.

Although we talk in terms of Days with ID you can just as easily use this term and price pattern to refer to bars or candles from any time frame chart.

The opposite to an ID is an OD.

This is a Toby Crabel price pattern.