Final answer:
Cosine180 degrees is a trigonometric function value, and it equals -1.
Step-by-step explanation:
The cosine of 180 degrees, denoted as cosine180 degrees, is a basic trigonometric function value. By referring to the unit circle or a trigonometric table, we know that at 180 degrees, the cosine function value is -1. Hence, the correct answer to what is cosine180 degrees is -1.