Final answer:
A standing wave is formed when constructive and destructive interference occur simultaneously, creating points of maximum amplitude called antinodes and points of no motion called nodes.
Step-by-step explanation:
The word that best fits the definition formed when constructive and destructive interference occur at the same time in a wave is a standing wave. In a standing wave, the locations where the interference is always constructive are called antinodes, and they are points of maximum amplitude. The locations where the interference is always destructive are called nodes, which are points of no motion. When two identical waves that have the same frequency and are moving in opposite directions superpose, they continually interfere constructively to form antinodes and destructively to form nodes, creating a standing wave pattern.