The whole category of sedges is more than just a little mystery to us, therefore instead of trying to find the name of this plant, we just went with the generic title. What we can say is this image shows the flower of a sedge, in its female form. The sedge will have a male form of the flower a little later on - the two don't occur on the same plant at the same time. Different plants vary in their flowering times, so that there is an overlap where one plant has is female flowers out at the same time when another plant is showing its male flowers.

The stem of this plant is nearly triangular with sharp edges, recalling the rhyme:

Sedges have edges; Rushes are round
Grasses are hollow to the ground