The sequence $x_0, x_1, x_2, \ldots$ is defined by the conditions \[ x_0 = a, x_1 = b, x_{n+1} = \frac{x_{n - 1} + (2n - 1) ~x_n}{2n}\] for $n \ge 1,$ where $a$ and $b$ are given numbers. Express $\lim_{n \to \infty} x_n$ concisely in terms of $a$ and $b.$

1. We start with the given recurrence relation:
   \[
   x_{n+1} = \frac{x_{n-1} + (2n-1) x_n}{2n}
   \]
   for \( n \ge 1 \), with initial conditions \( x_0 = a \) and \( x_1 = b \).

2. To find the limit of \( x_n \) as \( n \to \infty \), we assume that the sequence converges to some limit \( L \). Thus, we have:
   \[
   \lim_{n \to \infty} x_n = L
   \]
   and we need to find \( L \) in terms of \( a \) and \( b \).

3. If \( x_n \to L \) as \( n \to \infty \), then \( x_{n-1} \to L \) and \( x_{n+1} \to L \) as well. Substituting these into the recurrence relation, we get:
   \[
   L = \frac{L + (2n-1) L}{2n}
   \]

4. Simplifying the right-hand side:
   \[
   L = \frac{L + (2n-1) L}{2n} = \frac{L (1 + 2n - 1)}{2n} = \frac{L \cdot 2n}{2n} = L
   \]

5. This equation \( L = L \) is always true and does not provide any new information. Therefore, we need to analyze the behavior of the sequence more closely.

6. Consider the difference between consecutive terms:
   \[
   x_{n+1} - x_n = \frac{x_{n-1} + (2n-1) x_n}{2n} - x_n
   \]
   Simplifying this, we get:
   \[
   x_{n+1} - x_n = \frac{x_{n-1} + (2n-1) x_n - 2n x_n}{2n} = \frac{x_{n-1} - x_n}{2n}
   \]

7. As \( n \to \infty \), the term \(\frac{x_{n-1} - x_n}{2n}\) approaches 0 because \( x_{n-1} \) and \( x_n \) are both approaching \( L \). Therefore, the difference \( x_{n+1} - x_n \) approaches 0, indicating that the sequence \( x_n \) stabilizes.

8. To find the exact value of \( L \), we need to consider the initial conditions and the behavior of the sequence. Notice that the recurrence relation is a weighted average of \( x_{n-1} \) and \( x_n \). As \( n \) increases, the weight on \( x_n \) becomes dominant.

9. Given the initial conditions \( x_0 = a \) and \( x_1 = b \), and the fact that the sequence stabilizes, we can infer that the limit \( L \) is a weighted average of \( a \) and \( b \). However, since the weight on \( x_n \) becomes dominant as \( n \) increases, the limit \( L \) will be closer to \( b \).

10. Therefore, the limit of the sequence \( x_n \) as \( n \to \infty \) is:
   \[
   \lim_{n \to \infty} x_n = b
   \]

The final answer is \(\boxed{b}\).