That's pretty well the definition of "spent" for the purposes of this discussion.

Actually, it's more technical than that as you could have a transaction that spends coins from an address and sends them to the same address and it would (probably) still destroy some number of bitcoindays. What you're actually spending is one or more of what is commonly called a "TxOut" which is worth some number of bitcoins. An address can have zero or more spendable TxOuts. In order to spend a TxOut you need to supply an appropriate signature generated using the private key associated with that address.

ByteCoin