Original: His conjecture was completely proved by Chebyshev (1821–1894) in 1850 and so the postulate is also called the Bertrand-Chebyshev theorem or Chebyshev's theorem.
Simplified: His statement was completely proven by Pafnuty Chebyshev in 1850. For this reason, the postulate is also called the Bertrand-Chebyshev theorem or Chebyshev's theorem.