Even or Odd!

If $n$ is a positive integer or $0$ , then it always even. Because then $3^n$ is an odd number after minus $1$ , $3^n-1$ make an even number.

So, the answer is $\boxed{\color{#20A900}{\text{even}}}$

The last digit of $3^{n}$ is 3, 9, 7, and 1. It is easy to show that last digit of $3^{n}-1$ is 2, 8, 6, and 0. Clearly, the number with last digit 2, 8, 6, and 0 are divisible by 2.

Hence, $3^{n}-1$ is always even number.