  We present the ORIGAMI method of identifying structures, particularly halos,
in cosmological N-body simulations. Structure formation can be thought of as
the folding of an initially flat three-dimensional manifold in six-dimensional
phase space. ORIGAMI finds the outer folds that delineate these structures.
Halo particles are identified as those that have undergone shell-crossing along
3 orthogonal axes, providing a dynamical definition of halo regions that is
independent of density. ORIGAMI also identifies other morphological structures:
particles that have undergone shell-crossing along 2, 1, or 0 orthogonal axes
correspond to filaments, walls, and voids respectively. We compare this method
to a standard Friends-of-Friends halo-finding algorithm and find that ORIGAMI
halos are somewhat larger, more diffuse, and less spherical, though the global
properties of ORIGAMI halos are in good agreement with other modern
halo-finding algorithms.
