What is a session, do you need to be in the water a certain amount of time? , do you need to catch this many waves ? What if you go out catch two nice waves, in a half hour, the wind shifts and conditions are no longer rideable is that a session?
A "session" is a term used when you spend any amount of time in the water without bringing your friends. Thus, it could also be coined, "solo-session".
A "sesh" is more of a slang word referring to a surf session, involving more than one person - you and your buddies.
A "party sesh" is when almost every one of your bro's are out with you, almost so that it seem more like party than an actual sesh.
And after a very good sesh, I like to combine the factors of "sesh" and '"success" and use the word "succ-sesh!" But this may be misleading to a passerby because it also sounds like "suck-sesh," Which means the exact opposite.