The body nimbus perceptible to a sime caused by the presence of selyn, mathematically akin to the electric field surrounding an electron. (See also: Nager)  "Gradient" refers to the gradual fall-off of intensity with distance from the subject. 

(Source:  Index card fileSime Surgeon,  Describing Ilyana's field --- "a throbbing nimbus of energy."

A sort of pulling force --- like gravity which falls off with distance.  A difference between fields, the lesser tending to flow into the former.

(Source:  Index card file.  Operation High Time,  Possessed by both simes and Gens.  Gradient can be a field on its own, doesn't need 2 or more people.  --- "with no field gradient other than my own to distract me."