  (Abridged) Virial mass is used as an estimator for the mass of a dark matter
halo. However, the commonly used constant overdensity criterion does not
reflect the dynamical structure of haloes. Here we analyze dark matter
cosmological simulations in order to obtain properties of haloes of different
masses focusing on the size of the region with zero mean radial velocity. Dark
matter inside this region is stationary, and thus the mass of this region is a
much better approximation for the virial mass. We call this mass the static
mass to distinguish from the commonly used constant overdensity mass. We also
study the relation of this static mass with the traditional virial mass, and we
find that the matter inside galaxy-size haloes is underestimated by the virial
mass by nearly a factor of two. At redshift zero the virial mass is close to
the static mass for cluster-size haloes. The same pattern - large haloes having
M_vir > M_static - exists at all redshifts, but the transition mass M_0 = M_vir
= M_static decreases dramatically with increasing redshift. When rescaled to
the same M_0 haloes clearly demonstrate a self-similar behaviour, which in a
statistical sense gives a relation between the static and virial mass. To our
surprise we find that the abundance of haloes with a given static mass, i.e.
the static mass function, is very accurately fitted by the Press & Schechter
approximation at z=0, but this approximation breaks at higher redshifts.
Instead, the virial mass function is well fitted as usual by the Sheth & Tormen
approximation. We find an explanation why the static radius can be 2-3 times
larger as compared with the constant overdensity estimate. Applying the
non-stationary Jeans equation we find that the role of the pressure gradients
is significantly larger for small haloes.
