What is the "description" of a Turing machine?
The description is a string that encodes the formal specification of the machine, comprising:


the set of states,
the start state,
the set of halting states (or, where appropriate, the sets of accepting and rejecting states),
the tape alphabet,
the input alphabet,
the transition function.


Some authors also explicitly include the identity of the blank symbol in the description of the machine; others leave it to convention (e.g., stating that the alphabet will always be $c_1, \dots, c_k$ for some&nbsp;$k$, and $c_1$&nbsp;is the blank symbol).