The word 'Spendable' more precisely says what the balance actually means. It is not always 'confirmed' (for example in the case of spending unconfirmed change #3676).

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ACK
Are there any more places, where this needs to be change?
A quick git grep in src/qt and src/qt/locale didn't show up any other misuses of Confirmed. In other places (like the send tab) the spendable balance is just called 'balance' and is the only one shown, which is good too.
That's indeed a good thing, just merge then :).
"Available" might be better, to match existing online banking terminology.
If we rename "confirmed" to "available" then how about renaming "unconfirmed" to "pending"?
available and pending sound good to me
Nit picking :
Definition from Oxford : http://www.oxforddictionaries.com/us/definition/american_english/pending
to summarise : Pending - until (something) happens or takes place.
Pending is a preposition .. so it should be pending confirmation or pending something else it shouldn't be used on its own ..
My bank uses "Current Balance" and "Available Balance".
those terms are unoffensive to me or the english language ;) (ie +1 for lukes-jr's suggestion.)
Going to go with available (which implies it can be spent now and is really there) and pending (which implies that there is a time factor involved, which there is).
The word 'Spendable' more precisely says what the balance actually means.
Avoids the confirmed/unconfirmed confusion that can be caused by #3676.