Deadspace

Technically yes the dead space is the bit left in the vessel when emptied, whether that be a mash tun or boil kettle.
These days people are confusing it a bit because single vessel systems have what is called recoverable dead space because although the space under the bottom of the mash pipe screen is "dead space" to the grain while mashing, once the grain pipe is removed, that area is back into action for the boil.
Where it gets a little confusing is some time tweaking these values in software away from their actual values can help some calculations.
Is that all clear as mud? hhaha
Cheers!