unknown quantity

Từ điển WordNet v3.1 - WordNet Dictionary

noun

1. a factor in a given situation whose bearing and importance is not apparent (Freq. 1)
- I don't know what the new man will do; he's still an unknown quantity
• Hypernyms:
factor
2. a variable whose values are solutions of an equation
• Syn:
unknown
• Derivationally related forms:
unknown (for: unknown)
• Hypernyms:
variable