unknown quantity

An unpredictable person or thing, as in We don't know how the new pitcher will do--he's an unknown quantity. This expression comes from algebra, where it signifies an unknown numerical value. Its figurative use dates from the mid-1800s.

The noun has 2 meanings:

Meaning #1: a factor in a given situation whose bearing and importance is not apparent; he's still an unknown quantity"

Meaning #2: a variable whose values are solutions of an equation
  Synonym: unknown