"A currency is a unit of exchange, facilitating the transfer of goods and services." - Wikipedia.

"'Fictional Currency' is used as game tokens and have no monetary value in and of themselves and as a result are not be taxable as income. In most cases a fictional currency constitutes a limited license right for use by account holders and is subject to the terms of each Fictional Currency provider. The World Stock Exchange supports the "Linden Currency" (Linden Dollar, LND$, L$, Lindens) and the "World Internet Currency" (WIC, WICS, W$)." - LukeConnell Vandeverre, Chairman and CEO, World Stock Exchange.

Vandeverre is right ... and wrong when he says "have no monetary value". You see, as long as he sticks to first party tokens (W$, in this case) he's right. We're just talking game tokens. However, any interaction with Linden Dollars (L$) or other currencies messes up the statement.