  We use deep Hubble Space Telescope photometry of the rich, young (~20-45
Myr-old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its
stellar mass function (MF) down to ~0.15 Msun. This represents the deepest
robust MF thus far obtained for a stellar system in an extragalactic,
low-metallicity ([Fe/H]~-0.4 dex) environment. Combining our results with the
published MF for masses above 1.0 Msun, we obtain a complete present-day MF.
This is a good representation of the cluster's initial MF (IMF), particularly
at low masses, because our observations are centred on the cluster's uncrowded
half-mass radius. Therefore, stellar and dynamical evolution of the cluster
will not have affected the low-mass stars significantly. The NGC 1818 IMF is
well described by both a lognormal and a broken power-law distribution with
slopes of Gamma=0.46+/-0.10 and Gamma~-1.35 (Salpeter-like) for masses in the
range from 0.15 to 0.8 Msun and greater than 0.8 Msun, respectively. Within the
uncertainties, the NGC 1818 IMF is fully consistent with both the Kroupa
solar-neighbourhood and the Chabrier lognormal mass distributions.
