TbitLen(X) definition in Computer Security terms:
Definition(s): A truncation function that outputs the most significant (i.e., leftmost) bitLen bits of the input bit string, X, when the bit length of X is greater than bitLen; otherwise, the function outputs X. For example, T2(1011) = 10, T3(1011) = 101, T4(1011) = 1011, and T5(1011) = 1011.
Source(s): NIST SP 800-56A Rev. 2
reference: CSRC Glossary