Examination of available numerical evidence for known primes led Legendre to suspect that satisfies:
where B is Legendre's constant. He guessed B to be about 1.08366, but regardless of its exact value, B existing implies the prime number theorem.
Later Carl Friedrich Gauss also examined the numerical evidence and concluded that the limit might be lower.
Being evaluated to such a simple number has made the term Legendre's constant mostly only of historical value, with it often (technically incorrectly) being used to refer to Legendre's first guess 1.08366... instead.