The first step in the two-step key-derivation procedure during which a key-derivation key is produced. The second step in the procedure is key expansion.
Sources:
NIST SP 800-133 Rev. 2
The first step in the two-step key-derivation procedure specified in this Recommendation; during this step, a key-derivation key is produced from a shared secret.
Sources:
NIST SP 800-56C Rev. 2