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1) an as-yet-undiscovered theory unifying many scientific field theories into one understanding of how the universe operates

2) something unattainable, much as finding an actual unified field theory has baffled scientists for decades
Einstein spent the last two decades of his life trying to develop a unified field theory.
Person 1: John keeps asking Sarah out, but she's not biting.
Person B: She's his unified field theory. Ain't gonna happen.
by Michael Grant July 21, 2008