Security Level - Clearance or classification and a set of designators of special access approvals; in other words, a clearance and a set of designators of special access approval or a classification and a set of such designators, the former applying to a user, the latter applying, for example, to a computer object.
NIST IR 7298
Security Level - A hierarchical indicator of the degree of sensitivity to a certain threat. It implies, according to the security policy being enforced, a specific level of protection. SOURCE: FIPS 188