CH02::propagation: define full node / peer

ch02.asciidoc

@@ -477,10 +477,15 @@ network is to propagate transactions and blocks to all participants.
 
 ===== How it propagates
 
-((("Bitcoin nodes", "defined")))((("nodes", see="Bitcoin nodes")))Any
-system, such as a server, desktop application, or wallet, that
-participates in the Bitcoin network by "speaking" the Bitcoin protocol
-is called a _Bitcoin node_. Alice's wallet application can send the new
+((("Bitcoin nodes", "defined")))((("nodes", see="Bitcoin nodes")))
+Peers in the Bitcoin peer-to-peer network are programs that have both
+the software logic and the data necessary for them to fully verify the
+correctness of a new transaction.  The connections between peers are
+often visualized as edges (lines) in a graph, with the peers themselves
+being the nodes (dots).  For that reason, Bitcoin peers are commonly
+called "full verification nodes", or _full nodes_ for short.
+
+Alice's wallet application can send the new
 transaction to any Bitcoin node it is connected to over any type of
 connection: wired, WiFi, mobile, etc.  Her bitcoin wallet does not have
 to be connected to Bob's bitcoin wallet directly and she does not have