Security Strength

A measure of the computational complexity associated with recovering certain secret and/or security-critical information concerning a given cryptographic algorithm from known data (e.g. plaintext/ciphertext pairs for a given encryption algorithm).
SOURCE: SP 800-108

A number associated with the amount of work (that is, the number of operations) that is required to break a cryptographic algorithm or system. Sometimes referred to as a security level.
SOURCE: FIPS 186