The paper I read starts with the following text:
One of the most appealing open problems in the graph streaming area (see Problem 60 in [1]) is to close the gap between the approximation factor (i.e., lower bound) of 0.5 (achieved by a simple greedy algorithm) [10] and the hardness result (i.e., upper bound) of 1 − 1/e ' 0.63 [11, 14] for the classical problem of the maximum matching in the semi-streaming model.
I cannot understand what is approximation factor...
Thanks!