Technically, No.
Water in a glass (or other vessel) has a curved surface known as a meniscus.
A large body of water (like an ocean) actually follows the curve of the earth. So the surface of the water is a curve or section of a sphere.
Chat with our AI personalities
Surface tension.
Any, depending on the volume of water spilt and the area of the flat surface. The specific height is also constrained by the surface tension of the water, the atmospheric pressure above it, and the gravity below it.
A mirror with a flat surface - planar
Its base is a flat surface
A flat surface generally refers to a horizontal surface and is referred to as the plane.