What is a double spend?

A double spend happens when an attacker is broadcasting two different transactions to the network where the same funds are used. One transaction is to himself and the other one is to the merchant. The idea is that the attacker makes the merchant confident that he received the payment and ships the product, while ensuring that the transaction to himself will be included in the (longest) blockchain. Once the transaction to the attacker is permanently included, the other one to the merchant will be rejected or cancelled while the product is already shipped. The possibility or ease to double spend depends on the following factors: