The Chandra Source Catalog Release 2.1 (CSC 2.1) is the latest version of a virtual X-ray astrophysics facility that enables both detailed individual source studies and statistical studies of large samples of X-ray sources detected in ACIS and HRC-I imaging observations obtained by the Chandra X-ray Observatory. CSC 2.1 covers roughly 730 square degrees of sky and contains 407,806 unique X-ray sources and over 1.3 million individual source detections identified in more than 15 thousand Chandra imaging observations released publicly prior to the end of 2021. The catalog provides carefully-curated, high-quality, and uniformly calibrated and analyzed tabulated positions, significance, spatial extent, aperture photometry fluxes, hardness ratios, spectral model fits, and temporal variability in multiple energy bands. All properties have associated independent lower and upper confidence limits. Along with tabulated data, the catalog includes roughly 40 different types of data products (e.g., spectra and responses, light curves, photometric marginalized probability density functions, extended source region polygons) for individual and co-added observation detections and sources, enabling fast scientific analysis without having to perform extensive preparatory data processing. The compact source sensitivity limit for most observations is roughly 5 photons over much of the field of view, allowing us to reach the faintest sources in the X-ray sky. Catalog astrometry is tied to the Gaia-CRF3 realization of the ICRF for improved sky positions, with a systemic astrometric error of approximately 0.29 arcsec (95% confidence).