- was proposed by Bernhard Riemann (1859), and is a conjecture about the distribution of the zeros of the Riemann zeta function.
The Riemann hypothesis asserts that all interesting solutions of the equation
ζ(s) = 0
lie on a certain vertical straight line.
It remains unproved whether the hypothesis is true or false.
Full details at Wikipedia
An essay on the hypothesis can be found here here courtesy University of Washington