To find the fifth term of the sequence defined by the function \( f(n) = 2^n - 1 \), we need to evaluate \( f(5) \).
\[ f(5) = 2^5 - 1 \]
Calculating \( 2^5 \):
\[ 2^5 = 32 \]
Now, substituting back into the equation:
\[ f(5) = 32 - 1 = 31 \]
Therefore, the fifth term of the sequence is:
\[ \boxed{31} \]