In Unity, a coroutine is a special type of function that can be used to pause the execution of a function for a specified period of time or until a certain condition is met. Coroutines are typically used for animations, state machines, and other complex behaviors that require precise timing or asynchronous execution.

To define a coroutine in Unity, you use the `yield` statement to specify when the coroutine should pause and resume execution. For example, you could use the following code to create a coroutine that waits for two seconds before continuing:

IEnumerator WaitTwoSeconds()
{
    yield return new WaitForSeconds(2);
    Debug.Log("Two seconds have passed.");
}​

To start a coroutine, you call the `StartCoroutine()` method and pass in the name of the coroutine function. For example, you could use the following code to start the `WaitTwoSeconds()` coroutine:

StartCoroutine(WaitTwoSeconds());​

Once a coroutine is started, it will continue to execute until it reaches the end or encounters a `yield` statement that pauses execution. At that point, Unity will continue executing the rest of the game logic before returning to the coroutine and resuming execution. This allows you to create complex and responsive game behaviors without blocking the main thread or causing performance issues.