A Double Spend is an attack wherein the same cryptoasset is used in two distinct transactions.
This type of attack involves creating an initial transaction and then quickly creating a second transaction using the same coins. Only one of these transactions will be successfully included in a block by the network, leaving the other invalid. This kind of attack relies on a counterparty to the transaction accepting it with few or no confirmations and can be avoided by ensuring a minimum number of confirmations appropriate to the network. In the Bitcoin protocol six transactions is commonly accepted as a statistically safe threshold for the transactions inclusion into the blockchain.