  This paper introduces Colossus, a public, open-source python package for
calculations related to cosmology, the large-scale structure (LSS) of matter in
the universe, and the properties of dark matter halos. The code is designed to
be fast and easy to use, with a coherent, well-documented user interface. The
cosmology module implements Friedman-Lemaitre-Robertson-Walker cosmologies
including curvature, relativistic species, and different dark energy equations
of state, and provides fast computations of the linear matter power spectrum,
variance, and correlation function. The LSS module is concerned with the
properties of peaks in Gaussian random fields and halos in a statistical sense,
including their peak height, peak curvature, halo bias, and mass function. The
halo module deals with spherical overdensity radii and masses, density
profiles, concentration, and the splashback radius. To facilitate the rapid
exploration of these quantities, Colossus implements more than 40 different
fitting functions from the literature. I discuss the core routines in detail,
with particular emphasis on their accuracy. Colossus is available at
bitbucket.org/bdiemer/colossus.
