To find the 10th term of the sequence described by the formula \( a_n = 9n \), we need to substitute \( n = 10 \) into the formula.
\[ a_{10} = 9 \cdot 10 \]
Calculating this gives:
\[ a_{10} = 90 \]
Therefore, the 10th term of the sequence is \( \boxed{90} \).