Fifty mL refers to 50 milliliters in the metric system of measurement, which is equivalent to approximately 1 2/3 fluid ounces using the U.S. customary system of measurement. In recipes, 50 milliliters equals 1/4 cup.
A liter is the standard measurement for volume in the metric system. A milliliter is 1/1000 of a liter, and is used to measure small quantities. In recipes, measurements in milliliters are used for liquid ingredients throughout much of the world.
In several countries, including Russia, Israel and Brazil, 50 milliliters is the size of a single shot of liquor. This contrasts with the United States where 1.5 fluid ounces or 44 milliliters make up a single shot.