An angle that has a measure less than a right angle (less than 90 degrees)
In ordinary English, acute means "sharp," but metaphorically. Knives are not acute or obtuse -- only sharp or dull are used for that literal sharpness -- but when someone's wit or intelligence is sharp we might refer to them as "sharp" or "acute," and when they are dull, we might say they are obtuse. Similarly, pain can be acute when is sudden and severe (though we don't use obtuse when it is long-lasting or dull).
obtuse, right angle