Pour the vinegar into a small saucepan and bring to a boil over medium-high heat.
Reduce heat to medium or medium-low to maintain a simmer. Stir occasionally, until the vinegar thickens, coats the back of a spoon, and is reduced to about 1 cup - 20 to 25 minutes. I find the wider the pan, the faster this can happen.
Cool for at least 1 minutes before using. Store in airtight container in refrigerator.
Notes
Balsamic glaze can be stored in an airtight container in the refrigerator for a month! I like jars like these for storing sauces.