A full node is a software that performs full verification on everything, if it skips any part of the verification then it no longer can be considered a "full node".
Note that by this definition, there are relatively few bitcoin full nodes, since most nodes use the default nonzero assumevalid.
Good point. And how many nodes run at checklevel=4? And...
tromp, I am particularly glad to see you here.
From your long experience working both with Bitcoin, and with altcoin POW, what do you think of my prediction that the current push to POSify decent altcoins is ultimately designed strategically to isolate Bitcoin?
(I am very upset about what is going on with those altcoins. But first things first—here, I must focus on Bitcoin.)
Looking forward to your reply. 
For my part, I won’t waste my time debating with POS shills. That is why my first post in this thread announced: I am not here to debate.