A hill is, of course, a high area, from which the terrain slopes off toward surrounding areas. The term hill is most often used for a somewhat rounded feature that appears as a bump or knob on a land surface. It can be a small feature, on the scale of a football field, or it can be much larger, perhaps on the scale of a city, or part of a city. The term mountain should be reserved for truly mountainous terrain, but sometimes hills are called mountains locally. The following data is from the example used in the triangulation exercise. Scanning the z values, you will see the numbers increasing toward the general area of the 345 point, the point with the largest z value on the map:
Contours will run around a hill and will close to make "rings" around the hill. This makes hills easy to spot on a contour map. Individual contours are said to close, or to exhibit closure, when they are in a continuous path, or loop, regardless of shape:
The 150 contour in the lower part of the map does not close in the area of this map. It might come around and close in the area above the top of the map, or not (hills don't go forever -- they connect in some fashion to other landforms). However, with just the data shown on this map, we can't tell if the 150 contour closes or not.