This tag should be used for anything related to the secp256k1 algorithm used for Bitcoin's public key cryptography.

secp256k1 refers to the parameters of the elliptic curve used in Bitcoin's public-key cryptography and is defined in Standards for Efficient Cryptography (SEC) (Certicom Research, http://www.secg.org/sec2-v2.pdf).

Secp256k1 is a Koblitz Curve