In cryptography, security level is a measure of the strength that a cryptographic primitive — such as a cipher or hash function — achieves.
[3] This allows for convenient comparison between algorithms and is useful when combining multiple primitives in a hybrid cryptosystem, so there is no clear weakest link.
[9] The design of most asymmetric algorithms (i.e. public-key cryptography) relies on neat mathematical problems that are efficient to compute in one direction, but inefficient to reverse by the attacker.
Their security level isn't set at design time, but represents a computational hardness assumption, which is adjusted to match the best currently known attack.
The following table are examples of typical security levels for types of algorithms as found in s5.6.1.1 of the US NIST SP-800-57 Recommendation for Key Management.