A blockchain's economic security refers to how secure a blockchain is from economic forces (i.e., capital), as opposed to technological vulnerabilities. Quantifying economic security may refer to the amount of capital required to attack or compromise a blockchain. Economically secure systems are, therefore, those in which carrying out an attack would be prohibitively expensive. In some systems, this is considered the cost of breaking byzantine fault tolerance (BFT) or carrying out a 51% attack.
Importantly though, blockchains are rarely, if ever, secure completely from economic attacks; instead, 'secure' systems are those whereby an economic attack would not be financially rational, at least not directly. However, such attacks could still occur due to other motivations.