In Italian, the term means "narrow" or "close". In the Fugue, it designates the imitating voice which enters before the preceding voice has completed the theme. This produces the effect of increasing the tempo as well as the intensity of the passage.
Beethoven used this device in many of his works, as is clearly reflected at the end of his Fifth Symphony. There, the texture is not fugal, but the effect of the increased tempo is clear.
