I believe an alt is anything that has not proved it individuality from the rest of the established currencies. For this reason BTC, LTC and NMC I believe are currencies in their own right and not alts. I think a good definition would be alt = immature currency with no real difference from current ones.
How is NMC or PPC or TRC, etc "Immature"?. They are what the category states they are. Alternate Cryptocurrencies.
Definition of those 2 words:
Alternate:
n. (-nt)
1. A person acting in the place of another; a substitute.
2. An alternative.
-- Online Dictionary
Cryptocurrency:
A cryptocurrency is a type of digital currency (which in turn is a type of alternative currency) that relies on cryptography, usually alongside a proof-of-work scheme, in order to create and manage the currency --Wikipedia
In the definition of Cryptocurrency, wikipedia even goes on to say that
Cryptocurrencies are peer-to-peer and decentralized, and are based on the first cryptocurrency, Bitcoin.
Ripple is not "decentralized" as it is not controlled by the people (p2p) it is made and given out by the issuer (miner) which makes it the opposite of a p2p currency.