VOUT
The vector of an output in a bitcoin transaction.
A vout is basically an index number for an output in a transaction.
Usage
You can use a txid and a vout to uniquely select an output to use as an input in a new transaction.
Notes
In programming, counting starts at 0. So if we want to use the first output of an existing transaction, we put a vout of 0. (The second output would be 1.)
The āvā in vout stands for vector, because outputs in the bitcoin source code are stored in a vector data structure (which is basically an array).