1610s, "bright cloud surrounding a divine or sacred personage," from Latin nimbus "cloud," which is perhaps related to nebula "cloud, mist" (from PIE root *nebh- "cloud"). In art, the meaning "halo around the head of a representation of a divine or sacred person" is by 1727. Figurative use is by 1860. Compare aureole.
The nimbus of God the Father is represented as of triangular form, with rays diverging from it on all sides, or in the form of two superposed triangles, or in the same form (inscribed with the cross) as that of Christ. The nimbus of Christ contains a cross more or less enriched; that of the Virgin Mary is a plain circle, or occasionally a circlet of small stars, and that of angels and saints is often a circle of small rays. When the nimbus is depicted of a square form, it is supposed to indicate that the person was alive at the time of delineation. [Century Dictionary]