A narrow definiton will identify Protestants as those German princes who protested the Edict of the Diet of Speyer in 1529. By that edict, the Catholic majority of princes had sought to crush the reform movement by taking away from other princes the right to determine the religion in their regions. The evangelical princes protested and sought a unified political front against the Roman Catholic princes.
In the political sense, Lutheran princes were among those first Protestants. In the religious sense, however, Luther and his followers soon distinguished their confession from Zwingli, Calvin, and others chiefly over the Lord's Supper.
Thus, while dictionaries lump together all who sought to address the abuses of Rome as "Protestant," it is important to distinguish the political realities from the theological realities. If Protestant means all those denominations that teach a figurative or spiritual or symbolic view of the real presence, Lutherans are not Protestants.
"The Blessings of Weekly Communion" - CPH - pp. 174-175