Able to replace or be replaced by another identical item; mutually interchangeable. 

A cryptocurrency is fungible if any two tokens are identical. Bitcoin and TROPTIONS are fungible. Smart contracts generally are not (they are unique tokens that represent a specific transaction). In regular currency, dollars are fungible. But a 1933 Saint-Gaudens Double Eagle (of which there are only a few in existence) is non-fungible. Any two may have different values based on their conditions. Fungibility is important for exchange or trading.