In neuroanatomy, "gyrus" is the
generic term for those parts of the folded, bumpy cortex that are prominent and visible on the surface -- i.e., as Webster says, the prominent folds in-between the
grooves of the
cortex. (The generic term for these grooves is "
sulcus"). Far from being random and unorganized, the
location, size and shape of the gyri of the brain are fairly
consistent across all human brains, such that they can be identified and named in several ways.
The most straight-forward and rigorous naming convention is based on simply the location of the gyri. Each gyus is given a name based first on the lobe of the brain on which it is found (frontal, temporal, parietal, occipital, limbic) and second upon its location within that lobe (superior, inferior, medial, mesial, lateral,etc.). For example the superior temporal gyrus describes the gyrus located in the upper-most part of the temporal lobe (at the border with the parietal lobe.) They may also be named in reference to prominent brain land marks: for instance, the precentral gyrus and postcentral gyrus are named in reference to the central sulcus or central fissure, (a deep and prominent groove which runs in an oblique vertical line from the superior to medial surface of the brain.) They may also be named by their morphology, (e.g. Angular gyrus and Cingulate gyrus), though very often these names are given in addition to the more rigorous location-based names.
It is worth noting that this naming convention is most often used in the medical profession, (i.e. by neurologists, radiologists and neurosurgeons) because it is useful to specify brain anatomy in a clear and unambiguous way. In non-medical study of the brain, naming conventions which refer to cellular architecture or brain function are used more often than the medical naming convention described above -- for example, see Brodmann's areas.