A deep strike is a non-scout player unit more then 4 hops out from a player or neutral system.
The AI Spawns zombie ships from nearby warp gates in retaliation.
I do not know the numbers but it's based mostly off difficulty level I believe.
There is talk about having the deep strike scale up depending on how far out the player's ships are but at the moment distance does not matter, the AI responds at a set pace any time a non-scout ship is more then 4 hops out.
D.