Final answer:
The combining form that means straight is 'rect-', which differs from a 'ray' in mathematics, a straight line starting at a point and extending infinitely in one direction.
Step-by-step explanation:
The combining form that means straight is rect-. This is not to be confused with ray, which in mathematics specifically refers to a straight line that originates at a point and extends infinitely in one direction. Understanding the concept of a ray is important as it is a fundamental element in geometry, representing an endless path in one orientation from its starting point.