What keyword is used to start defining an animation loop in CodeHS?

The keyword used to start defining an animation loop in CodeHS is "function." In programming, particularly in the context of creating animations, a function serves as a foundational building block that allows you to encapsulate the code needed to perform a specific action or set of actions, such as drawing an animation frame or updating the state of an animation.

When defining an animation loop, you typically create a function that contains the code for what should happen during each frame of the animation. This function can then be repeatedly called to create the illusion of motion or change on the screen. The loop itself is often managed by the system or framework outside of this function, which calls it at regular intervals.

The other choices do not serve as the primary keyword for defining the loop: "loop," "repeat," and "animate" may capture the concept of looping or animating but are not the syntactical keywords used in CodeHS to initiate the definition of an animation loop. Thus, understanding the role of functions is crucial in structuring an animation within the programming environment.