Bearishgirl, everyone has their own definition. Here’s one courtesy of Investopedia:

A term that refers to real estate properties that are either in foreclosure and have not yet been sold or homes that owners are delaying putting on the market until prices improve. Shadow inventory can create uncertainty about the best time to sell (for owners) and when a local market can expect full recovery. Also, shadow inventory typically causes reported data on housing inventory to understate the actual number of inventory in the market.

Corelogic defines it as homes seriously delinquent, in foreclosure and owned by lenders.

Standard and Poor claims it is all delinquent homes, not just REOs.

This obviously doesn’t help with determining any kind of precision in numbers, and it probably follows the more complex the definition, the more wild the variances in guesstimates.

The OP may be better served with defining his/her own goal in conducting the research, and then zeroing in on data that is most relevant to it.