One of the several species of the genus Viola which also includes pansies.
All Viola have five petals, and most violets are deep purple in color, often turning to white and/or yellow at the center. Some varieties just have white or yellow petals or petals of each color.
African violets (genus Saintpaulia) and dogtooth violets (Erythronium dens-canis) are not true violets but may be tagged here as well.
See pansy for how to distinguish that flower from violets.