We report a laser with record frequency stability, based on a 21 cm cryogenic silicon cavity. For best short-term stability, the cavity operates at 6 K to minimize Brownian thermal noise and achieves a fractional stability of 7×10⁻¹⁸ between 2 to 50 s of averaging time and 2.5 mHz linewidth. For best long-term stability the cavity operates at 16 K, where silicon has a zero-crossing of thermal expansion. Slightly increased thermal noise limits the performance to 9×10⁻¹⁸, consistent with the expected temperature scaling, but the long-term performance is improved. The laser then has a linewidth of 2.9 mHz and long-term frequency drift less than 4 μHz s⁻¹. The laser is used to enhance the performance of a state-of-the-art Sr lattice clock, operating with Rabi and Ramsey interrogation times beyond 10 s.