The description of the word you requested from the astronomical dictionary is given below.
plage = [French] beach
Plage is an area on the Sun that is brighter than its surroundings if one looks at it in the middle of a spectral line. The greater brightness means that relatively small magnetic flux tubes stick through the surface there. Plage is invisible in images taken in the continuum, except close to the edges of the solar disk, but even there the contrast between plage and its surroundings is quite small.