If the update expression is omitted in a for loop, the loop will execute indefinitely. This means that the loop will continue to run forever until a break statement is encountered or the program is terminated. Let's break down why this happens:

1. Initialization:
In a for loop, the initialization expression is executed only once at the beginning of the loop. It is used to initialize the loop control variable to a starting value. For example:

```
for (int i = 0; i < 5;="" i++)="" />
// loop body
}
```

In the above code, the initialization expression `int i = 0` initializes the loop control variable `i` to 0.

2. Condition:
The condition expression is evaluated before each iteration of the loop. If the condition is true, the loop body is executed. If the condition is false, the loop is terminated. In the example above, the condition `i < 5`="" is="" checked="" before="" each="" />

3. Update:
The update expression is executed after each iteration of the loop. It is used to modify the loop control variable. In the example above, the update expression `i++` increments the value of `i` by 1 after each iteration.

What happens if the update expression is omitted?
If the update expression is omitted, the loop control variable will not be modified. As a result, the condition of the loop will always remain true, leading to an infinite loop. The loop will continue to execute indefinitely without any way to terminate naturally.

For example, consider the following code snippet where the update expression is omitted:

```
for (int i = 0; i < 5;)="" />
// loop body
}
```

In this case, the loop will execute indefinitely because the value of `i` will never change. The condition `i < 5`="" will="" always="" be="" true,="" and="" the="" loop="" will="" keep="" repeating="" />

How to avoid an infinite loop?
To avoid an infinite loop, it is important to include the update expression in the for loop. This update expression should modify the loop control variable in a way that the condition eventually becomes false, allowing the loop to terminate.