  The splashback radius, $R_{\rm sp}$, is a physically motivated halo boundary
that separates infalling and collapsed matter of haloes. We study $R_{\rm sp}$
in the hydrodynamic and dark matter only IllustrisTNG simulations. The most
commonly adopted signature of $R_{\rm sp}$ is the radius at which the radial
density profiles are steepest. Therefore, we explicitly optimise our density
profile fit to the profile slope and find that this leads to a $\sim5\%$ larger
radius compared to other optimisations. We calculate $R_{\rm sp}$ for haloes
with masses between $10^{13-15}{\rm M}_{\odot}$ as a function of halo mass,
accretion rate and redshift. $R_{\rm sp}$ decreases with mass and with redshift
for haloes of similar $M_{\rm200m}$ in agreement with previous work. We also
find that $R_{\rm sp}/R_{\rm200m}$ decreases with halo accretion rate. We apply
our analysis to dark matter, gas and satellite galaxies associated with haloes
to investigate the observational potential of $R_{\rm sp}$. The radius of
steepest slope in gas profiles is consistently smaller than the value
calculated from dark matter profiles. The steepest slope in galaxy profiles,
which are often used in observations, tends to agree with dark matter profiles
but is lower for less massive haloes. We compare $R_{\rm sp}$ in hydrodynamic
and N-body dark matter only simulations and do not find a significant
difference caused by the addition of baryonic physics. Thus, results from dark
matter only simulations should be applicable to realistic haloes.
