It's actually called 'a spade',(noun), e.g. Dig out a spade of earth. Problem then is - how deep is that? Well, it's a spade's depth. So it all seems a bit pointless, cos' your back where you started.
I agree with hammerman, definately a spit. Its an old term used especially in digging a plot or bed,where trenching or double digging is used ie take out a trench one spit deep, fork manure into 2nd spit, then turn next spit on top of that, repeat this process until you reach the end of the plot and put soil from first trench into last trench. A great way of increasing the depth of your top soil.