Pour vinegar into saucepan and turn heat to medium-high. Bring the vinegar to a boil then turn down to a simmer.
Simmer until vinegar is reduced by about half, stirring occasionally. Take care not to let it scorch. Remove from heat and allow to cool. The glaze will thicken more as it cools and more once in the refrigerator.
Notes
The reduction time can vary based on the quality of balsamic vinegar and the size of the pan. Higher quality balsamic vinegars tend to be thicker because they have aged longer. A pan with a larger surface area will cook the reduction quicker than a smaller saucepan.