The structure of the leading-edge vortex from a delta wing at high angle of attack is addressed using a scanning laser version of high-image-density particle image velocimetry. Emphasis is on the global patterns of instantaneous vorticity. These patterns are related to distributions of averaged and fluctuating velocity and vorticity. At low angle of attack, in the absence of vortex breakdown, it is possible to detect a total of five distinct layers of vorticity; they all exhibit small-scale concentrations of azimuthal vorticity. Immediately downstream of the trailing edge of the wing, larger-scale vorticity concentrations appear in the outermost vorticity layers. At sufficiently high angle of attack, vortex breakdown evolves from the innermost two vorticity layers. For all of these classes of vortical structures, the values of dimensionless wavelength and circulation are assessed. Moreover, the onset of vortex breakdown is interpreted in terms of both instantaneous and averaged patterns of velocity and vorticity. These considerations lead to a direct comparison of vorticity-based and stagnation criteria for breakdown. In turn, these criteria are linked to the onset of regions of fluctuating vorticity in the initial region of breakdown.