The Merriam-Webster Word of the Day for August 25, 2025 is undulant. Undulant is an adjective used to describe things that exhibit a wave-like motion or form. It can refer to a wavy outline or surface. The word is useful for describing natural phenomena like the rolling motion of the sea. It can also be applied to landforms such as hills. Furthermore, undulant can characterize the movement of something like a snake. It is also suitable for describing a fluctuating state, such as a fever. The term originates from the Latin word "unda," meaning "wave." Related English words derived from "unda" include inundate and undulate. This adjective provides a descriptive way to capture a sense of undulating patterns.