Application == session == connection in DB2?
For the most part, application == connection == session when looking at it from DB2's perspective.  

Someone could probably make an argument that these aren't always necessarily the same in environments using connection concentrator or the database partitioning feature (DPF), but for all intents and purposes it's safe to use the terms interchangeably.  

You could also make the argument that, from the client-side, a single application may open multiple connections (i.e. a connection pool), but this is dba.stackexchange.com :-)